Most of the time, I only will trade for a player if he is already on the CPU's trade block. The exception would be if I have multiple first rounders, and a notable hole on my roster that I wasn't able to fill through free agency. Then, I might try to trade a first rounder plus more for a good player on a CPU team.
As for trading away my own players, in a "canon/serious" I put them on the block and take an offer that I like. If I'm being serious, then I generally only trade a guy away if he's in the last year of his deal and I can't resign him at a fair price.
Sometimes though, I'll see if I can start close from scratch with a club in a new franchise. In that case, I mess around and see what best trades I can get, ending up with a boatload of picks in the next draft. Wholly unrealistic, but fun in its own way.
Anyone who claims to be a fan of two teams in the same pro sport is actually a fan of none.