That's the point, there. These are unwanted and senseless bids from managers who don't/can't appreciate the outlay that we would have to make to replace the players they are making these derisory offers on. If we needed to sell the players then they would be on auction in the TM, not holding out for any tokens we might [not] get from negotiation...
If I have a player that is eligible for the Nego list and I get an offer on him, I might sell - depending on whether I can afford to release the player, the offer, and whether there's any ready replacements in the TM. A couple of times I've been able to buy a replacement in the TM (so I know how much I've spent), and then evaluate the offer and either accept, or set a selling price, turning the decision back to the prospective buyer.
It's about the only thing I'm pro a messaging system for - oh for the ability to send them a message saying 'are you <censored> serious????'