Presumably you can put up a player for auction at any point, not just if you have 14+ players and not just in the round subsequent to when you bought them?
Is there a deadline each round for letting you know which players you'd like to throw into the auction?
Also, what happens to a player that you put up for auction on which there are no bids? Does he return to your squad as if you'd never put him up for auction? Would that block you from picking up another player in a round where you were allowed to bid?
You can sell a player whenever you like. Not through the blind auction though. You could just post to say 'Anyone want Player X for £5million'. If someone replies and says 'I'll give you 3million' and you agree to sell, the deal is done.
Alternatively, you could hold your own blind auction through pms, or advertise players for sale and ask for pms etc. You might even bid for players not advertised etc. there are no restrictions. Just confirm in the selection thread when trades are completed.
If you try and fail to sell and your squad is full, you can't bid in the blind auction, unless you dispose of a player for nothing into the pool.