I think i figured out why:
by 3rd to 4th season, most computer teams has already done enough transfers so that they:
-have enough players on each position
-absolutely no money for transfer
using this glitch, what you can do is, say if you want to sell a goalie, you go into editing mode, and make every goalie play some other position but not goalie on one team, then the chance of that team negotiating you for a goalie increase.
I did this to a forward, but you have to keep your players affordable. I sold one of my strikers who is 500 yearly salary using this technique.
becareful as some teams just don't have the money for transfer, so pick teams you know who wins a lot.