I get the not wanting to sell good players, but really if an offer comes in that values the player at more than you think he's worth, it would be foolish not to take it.
If you're at McDonalds, and just as you're about to take the first bite of your Big Mac, someone offers you £10 for it, you're going to take it. You're not going to say "Um I dunno. If I take your £10, they might run out of Big Macs before I get to order another one."
And if they do run out, you could stay hungry and come back tomorrow, safe in the knowledge you got a good deal. Or, instead of a Big Mac, you could order a few quarter pounder with cheeses instead. Or you could head down the road to Burger King with your new tenner and get yourself a couple of XL bacon double cheeses instead.