You can't sell 2 leagues below or you can't buy 2 leagues above you.
So you can only sell 2 star players...If you have a 3-4 star player, you either let him go or, if he's young, keep him for a couple of seasons until he gets to 2 stars. Only then you'll be able to sell him. This could work with a larger team (dunno 26/28 players) but with 22 players it's just impossible ;-)
Pretty sure I've had level 10's bid against me, and I'm level 12.
I may be wrong, but I am SURE that the limit is 2 levels, just looked at my news section and one of my youth players I sold is playing for a level 10 team.
I don't claim to be a Top Eleven expert but, if anyone would like advice, please don't hesitate to contact me through 'private messaging', I try to reply as soon as possible.