Sports Simulation AI

What do you mean by trade? Buying and Selling Players?

The trading of one or more players from one team to another for one or more players.

At the moment, this isn't an AI problem. It's a domain modelling problem. You need to look at the real world example you're trying to model and formulate a set of rules or guidelines that capture that knowledge. Once you've done that, then you can look into what sort of AI algorithm might implement them.

Evaluate the relative utility of each player. This is based not only a relative skills of the players but also of the needs of the team to fill spots. For example, it doesn't do you any good to have 6 of the most skilled first-basemen in the league since you generally only need a starter and someone else that can fill in.

Once you list all the considerations, you can build a mathematical model for the needs of your team and the evaluations of the players - both yours and the trade prospects. (Oh yeah, toss salary in there somewhere, too.) The list will sort itself out by who is expendable and who is needed.

Of course, this is all rather complex... if someone can recommend a good book on the mathematics of utility theory to this fine OP, I'm sure it would help.

Treat it like real life. Each player has a perceived value. Each team has a budget. Each team has a goal (if the team is one of the top teams they will look to pick up a good player to improve the team in the short term, if the team is not as good, they might want to trade good veteran players for some young prospects in order to rebuild). By assigning teams budgets, mentalities (which change based on performance), and designing what type of players are good for teams with different mentalities. From this whether the trade can be accepted comes down to budget and fit (see InnocuousFox' advice).

That's what I'd do anyways. You can even extend this to free agents.