Everybody knows that to make a profit, you buy low and sell high. In fantasy baseball, to make a profit, you sell your mid-level players when their trade value is high and you buy stars when their value is low.
To get one of those stars, you make it look like the other guy would be better off without that star and you relieve him of that albatross around their neck that might otherwise be there for the last four months of the season. You always talk down about those stars and how they are getting old or how injuries are just dragging them down. By talking down players like Jose Reyes, Grady Sizemore and Josh Hamilton, maybe you can find an owner who expected more from their first round draft pick. Of course, guys like this are a proven commodity and will recover and post good numbers over the remainder of the season. You just have to get him before he starts to show signs of his old self.
To sell your overachieving mid-level player, you talk about how he has finally come into his own and will likely play at this level for many more years. Players like Torii Hunter, Marco Scutaro and Aaron Hill come to mind as guys playing better than they likely will in a month. By selling high right now, you can make a nice profit.
An added bonus is packaging the two together, but that might be asking a little much. Of course, always be willing to throw in a little extra to get that star on your team, they are difference makers and you want them to make that difference for your squad.