Here's a thought... how about letting users trade cards directly?
Not only would it be more thematic for a game with baseball cards, it would create more interaction between players. Deckbox.org or some other MtG trading site would be a good model for the kinds of features that would be needed to implement this well.
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Primary team:
Secondary team, with heavy restrictions on using the auction house:
Tertiary team is AFK... I'm not allowing myself to make any changes to the team except during the offseason:
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