I think that player for player transfer is good. Especially when you don't have enough money. for example you have too much defenders and you need a forward.

to prevent cheating, you can allow this transfer only if every player is not worth more than twice the other player value. also the team with the weaker player must pay the other team at least the difference between the market values.

I think you need to make it available only between seasons, because in the mid-season every player receives new 20 games where he can gain experience.

The experience would be a reason not to allow it in mid-season since it would give 20 new experience matches it would be very good to swap all your players with a friend.

There are many other ways this can be used to cheat too so no, we won't consider this. All player transfers must be through the open market.

you can make this available only in the end of the season, and one time per season for each team.

It's very useful for teams that can't buy players because of the high demand of players.

There are too many ways to cheat with this. If you're willing to trade in an equally good player then what's the point?