Valencia will definately not want to sell him, but if Chelsea made a sensible/serious offer, much more than the reported £15m, then Valencia would have to give the offer serious consideration, they are not that financially secure at the moment and are building a new stadium, so a stupid offer would certainly turn their heads. BUt I agree, they REALLY wouldn't want to part with their star striker.
Also worth pointing out, I don't think the player would be that keen on a move away from Spain, he is one of those Spaniards that need to be close to their surroundings, and although Chelsea could offer him millions, he is already on a very good contract.
but ....
Villa has had several fallings out with the Valencia coach Quique Flores, the latest of these bust-ups, two weeks ago, led to Villa being sent home from training, and being suspended from the next days training, leading to suggestions in Spain, that the player has become too big for his boots.
He is though, a brilliant player, much better than the over-hyped Torres.