I am not sure whether I like the player market idea. On a first glance, it sounds good. But it probably comes the same problems as all in-game-stores/auction houses.
a) In the worst case, you would be able to pay for players with real money, either directly im-game or by a making a deal on ebay and then trading in-game for a token amount. Worst case scenario, as I said.
b) All-powerfull teams won't have to worry about losses so much anymore. Throw your ClawPOMBers everywhere without protection, if they get fouled and die, well, there are plenty more for sale on the market.
c) It seems to me that by making arranged matches with friends you can produce ST+, AG+, doubles players quite easily. Need a Ratogre with Pro/Block/Claw? Just do a series of arranged matches with a friend and have him produce one, then buy it from him. There is probably not going to be a shortage of excellent players on the market.
All in all, in the best case it probably breaks balance in the worst it makes anything but private leagues, where you disable the market, unplayable.