I would pick and choose carefully on some cards to sell. Also remember your going to be sniping against a ton of players and you might get lucky and get players back for cheap but it's going to be a grueling grind and will take some time.
I would write down who you sell and what you got for the card back and have that list in front and have an idea of what you want to spend to get back so you have a profit margin for each player.
Most people find it silly but I actually have a notebook dedicated to madden for buying and selling so I can get a good return and not guess on what I may have bought a card for and sold and also you can figure in the 10% deduction as well and this can help out a lot.
Best advice I can give you. Hope it is somewhat helpful.