Im sort of two faced on this issue. While I wouldn't sell a good catch dog that works the way I like. I also wouldn't pay much for one either. Mostly because while I feel lucky to own a good one, why would I spend a lot of money on a dog that has a risky job.
If I truely care about a dog but I think that it isn't quite working the way I want or we just don't have the room anymore, I wont sell it, I will give it away to someone I trust to give it a good hunting home.
I once gave one away that I liked and worked for me to a good friend just to make room for a new dog, but turned out that he was a one owner dog so we have him back now and he is working as good as ever.
Sorry if I got off track.