Enter the 'Lineup' menu and select a player to sell. Put him into the reserves by pressing B and selecting the 'Move To Reserves' option. Then, press R to enter the 'Comparisons' screen. Select the player you want to put into reserves, then press B and choose a player you want to buy. Then, press X to swap. When the swapping screen appears with the clock, try to get there football to go past the 0 in the middle. When they do so, move your football on top of theirs. You will now swap players and they will have to pay you.