The trade system was flawed in UFC2. I didn't trade anything due to the ridiculous cost of it. It was way easier to just buy packs given you have little control over the outcome of the trade.
In UFC3 it's still flawed as you need gold fighters for each coin, which is a rare commodity you don't want to give away (unless you play a lot).
I still play the mode as I just want to fight AI and I'm tired of career but I'm done trading stuff in as the chance you get a usable move is very, very slim once you've got golden moves everywhere (which isn't hard at all: I got it and I haven't even played much).