The way I look at it is there is nothing wrong with getting a bargain, be it at a fair, a boot/ garage sale or from the vet or the vets family. However it is not on to then rub it in the person you got it offs face or the faces of other collectors or those who may have missed out on said item.
Another thing that I am beginning to notice is so called "dealers" and/ or "collectors" coming up to stalls and purposefully rubbishing an item in an attempt to get it cheaper. Another good one is "I can get this cheaper from such and such a place." This has happened to me before and I must admit I am not overly polite with these people. 1- I think if they are taken in good humour they may try it on again and 2- By essentially telling them to p**s off, I hope that they will be less inclined to try similar with other people.