How many teams think they can seriously compete at this time of the season? I think 12 can be classified as sellers. It should be 14, but the Angels and Dodgers aren't going to sell. The Dodgers will probably land Cliff Lee for the rest of their farm system, and still miss the playoffs.
The Marlins, Astros, Brewers, Blue Jays, Mariners, Mets, White Sox, Phillies, Royals, Twins, Padres, and Cubs are not going to compete for anything. The Blue Jays bet big with a bad hand, so they might be committed to see things out, and the Royals bet big with an even worse hand. It's a shame, because they could make some of the better pitchers available. It would be interesting to see what they could fetch for Dickey, but they probably won't trade him. The Royals could try to salvage something out of the Shields trade, but also doubtful. The rest of the potential sellers, with the exception of Cliff Lee, don't have any front line starters. They have some interesting depth options though.
The Astros, Brewers, Padres, and Mariners have no one anyone wants. The Marlins will probably trade Nolasco and Slowey. The Mets could make Marcum available. The White Sox have Peavy and Floyd. The Twins have Kevin Correia. The Cubs have the most to offer in Garza, Feldman, and Villanueva.
Those are probably the options that we are looking at. Lee will cost too much in prospects and money. I would see what it would take to land Peavy. He pitches pretty well in the AL and in a bandbox. He's cheaper than Lee, and we'd control him for two more years. He's also a better third option in the postseason than Lackey. If that falls through, I'd look at renting Garza, Feldman, or Marcum.