I've had some success acquiring players in negotiations.
Fact is, you have to overpay for them or you're not going to get them.
Think about what it would take for you to sell YOUR player in negotiations.

So here's what I do: I check out the team of the player I'm interested in (click make offer, and the manager's profile appears on the right)
If they're within a couple levels of you, forget it. Look for teams several levels higher than you - that 5* player to you is going to be a 2 or 3 star player to them. (Again, put yourself in their shoes - sell your first team player? no way. Sell your bench warming 2* that you just haven't gotten around to dumping AND getting a couple tokens for him? heck yeah!)