Using Baseball Prospectus' playoff odds, you should be able to tell who has the best chance of doing just that. Teams really should take those odds to determine whether they should be buyers and sellers because more often than not, they are right.
After games being played on July 6 last year, five of the ten eventual postseason teams had a 80 percent chance of making the playoffs or better. Every team that did have an 80 percent or greater chance of making the playoffs on this date last year did.
The team with the highest percent chance to make the playoffs on July 6 that ultimately didn't was the Milwaukee Brewers, who had a 71 percent chance to punch their ticket, but collapsed down the stretch and failed to make it.
Only three (Orioles, Royals, and Pirates) had less than a 50 percent chance of making the playoffs following action on July 6. By July 31, the Orioles were up to a 71 percent chance, the Royals were at a 17 percent chance, and the Pirates were at a 46 percent chance.
Knowing this, I will use Baseball Prospectus' current postseason predictions to determine who should buy and who should sell at the 2015 Trade Deadline.
All In (85% or greater)
St. Louis Cardinals (99.3%)
There's no reason why the Cardinals, who own MLB's best record at 54-28, should consider selling. In fact, Baseball Prospectus says that they have a 99.3 percent chance of making the playoffs, leaving just a very small chance for an extreme collapse. If the Cardinals play just .500 ball the rest of the way, that would put them on pace for 94 wins, which would definitely put them in prime position to punch their ticket to the postseason. The Cardinals have no reason to do anything but buy.
Los Angeles Dodgers (92.9%)
The Dodgers have the second-highest playoff percentage in the league, and nothing suggests that this team won't buy at the trade deadline. I predicted them to go out and get Johnny Cueto, perhaps the best pitcher available not named Cole Hamels. The Dodgers are always willing to spend money and prospects to make their team better and can easily justify doing so at the deadline.
Washington Nationals (85.6%)
The Nationals have arguably not played their best baseball yet, but still have an 85.6 percent chance to make the playoffs. They could use some reinforcements in their bullpen, but most of the additions the Nationals will be getting will be players coming back from injury, such as Jayson Werth, Ryan Zimmerman, and Stephen Strasburg. They may not need to make a ton of moves in July, but if they do, they have good reason to do so.
Houston Astros (84.7%)
The Astros have already shown interest in some of the top pitchers that will be available, and Baseball Prospectus' playoff odds completely backs that up. Houston's playoff percentage, at 84.7 percent, is so close to 85 percent that I had to shove them into this category. The Astros have showed that while they are a bit of a surprise, they still can win ballgames and do it over a period of time. The Astros by no means are "pretenders," as they have been able to hold a comfortable AL West lead since late April.
Should be buyers (70-85%)
Pittsburgh Pirates (81.3%), Chicago Cubs (73.1%)
The Pirates and Cubs have a case of some serious bad luck. They are playing in the National League's toughest division and would be either leading or close to leading any other division in the league. The only reason I'm wary of either of these teams buying at the deadline is because they are not division leaders and have more of a chance to collapse and fall out of the playoff race altogether. Unless they believe they have a real shot at the Cardinals (which it appears they don't), they shouldn't completely unload their farm systems to go out and get the best guy on the market.
Kansas City Royals (75.3%)
Of the three teams in this category, I am most comfortable with the Royals buying at the trade deadline due to the fact that they lead their division by a comfortable margin and need just one or two pieces to really put the pressure on the rest of the division. All signs point to the Royals making the playoffs again this year, so I would go ahead and pencil them in as buyers at the deadline.
Los Angeles Angels (64.8%)
The Angels are in a good position right now. They are playing good baseball and have shown the need for an upgrade in left field. Of the teams listed in this section, I truly believe they are the best and most complete team, so therefore they should be buyers at the deadline. The Angels could use some rotation help as well, but if they patch up a few spots, they will get into the postseason. My verdict? They should be buyers.
AL East: New York Yankees (59.4%), Toronto Blue Jays (39.6%), Tampa Bay Rays (34.9%)
The American League East division is lumped together because the division is so muddled and close that really anyone could win it. Baseball Prospectus' simulations give the Yankees the best chance to go to the playoffs out of that division, but with some rotation help, the Blue Jays are the division's best team. The Yankees and Rays should stand pat or make small moves at the deadline, while the Blue Jays should go out and make a splash for a rotation piece.
Detroit Tigers (36.7%)
With Miguel Cabrera being sidelined with his hamstring injury, I'm going to pencil the Tigers in as should be sellers, but as this team continues to try and make a run once again, they will find themselves trying to buy. The Tigers are heading towards a Phillies-esque fall, and if they don't realize that soon, it could only get worse if they decide to buy at the trade deadline in hopes for one last run at the World Series.
Maybe/Stand Pat (20-30%)
New York Mets (28.9%)
Even if the Mets added an offensive piece, I don't think that would be enough to get them to the playoffs this season. With that said, however, I could see them dealing for a guy with more than one season of control, as their young and talented pitching staff comes into their own. The Mets couldn't justify buying for a rental player, but a guy who is at least signed through 2016 could make sense.
Baltimore Orioles (28.7%)
The Baltimore Orioles have a ton of free agents at the end of the season that they probably should move. The Orioles could be one of those teams that tries to get 25-man roster guys with more years of team control in return. The Orioles could be a team that buys and sells at the trade deadline, and I would be fine with that.
San Francisco Giants (22.9%)
It's an odd year. The Giants aren't good enough to win the NL West, and considering that they have to deal with the Cubs and Pirates for the Wild Card, it will be tough for them to really make a run into the postseason. However, they still have a good core group of guys and the team has proved me wrong before. They probably should stand pat.
Shouldn't buy (Less than 20%)
Minnesota Twins (18.2%)
The Twins just aren't that good. Sure, they had a good run earlier this season, but all the numbers suggest that they were going to fall out of first in the AL Central. The Twins should really try and go for 2016, when some of their rookies will be more polished.
Texas Rangers (15.0%)
While the Rangers shouldn't buy, they probably will, as I consider them to be in a similar boat as the Tigers are in. The Rangers could legitimately contend, but they would more than a couple of upgrades, to the point where they probably shouldn't go for it this season.
Cleveland Indians (13.5%)
The Indians were a popular postseason pick prior to this season, but Baseball Prospectus' simulations show that they would need some serious luck to actually get there. The Indians shouldn't sell any pieces other than the impending free agents because my gut says that they will be back in the postseason sooner than later.
Boston Red Sox (11.8%)
The Red Sox are in a tough position right now. It might not be time for a fire sale quite yet, but it's definitely not time to go out and try and contend this season.
Seattle Mariners (7.0%)
The Mariners have had some issues staying in the race this season, and while they shouldn't sell off their entire team, they really shouldn't be buyers either.
Oakland Athletics (6.8%)
The Athletics are already shopping their pieces and it looks like they will be sellers.
Arizona Diamondbacks (6.4%)
The Diamondbacks are a team that should stand pat. They still have pieces to contend in the near future and as their pitching improves with guys coming back from injury, they could be a legitimate contender coming 2016.
Atlanta Braves (3.8%)
The Braves, especially in the offseason, have committed to becoming a selling team. They don't have any exciting pieces, but even though they have kind of surprised, they should by no means buy.
Chicago White Sox (3.2%)
The White Sox are in a tough position. They reportedly won't have a fire sale, which makes sense considering how much money they spent in free agency, but they need to get rid of Jeff Samardzija and still be planning to try again in 2016.
San Diego Padres (2.7%)
The Padres could buy at the deadline, but in all reality, they shouldn't. They've got some important games coming up that they need to win if people start seriously seeing them as contenders. Once again, they are proving that the winners of the offseason don't necessarily win during the season.
Miami Marlins (1.6%)
The Marlins shouldn't go into a fire sale, but Mat Latos, Dan Haren, Brad Hand, and other free agents at the end of the season should be gone.
Cincinnati Reds (1.1%)
The Reds have Johnny Cueto, Mike Leake, and possibly Aroldis Chapman available, and while they won't sell until after they host the All-Star Game, I expect it to come, as it should.
Colorado Rockies (0.2%)
I'd pull the trigger and deal Troy Tulowitzki. It's time for a real change in Colorado if they want to be relevant down the road.
Milwaukee Brewers (0.2%)
The Brewers should enter into a fire sale.
Philadelphia Phillies (0.0%)
Now, these rankings and categories don't mean that each of these teams will do as I advise. Their postseason percentages could change and perhaps an addition is all they need to do that. However, Baseball Prospectus' odds are very accurate and should not be taken lightly. Teams really should use them to determine whether they could justify buying at the deadline.