Quick answer: I'd actually pay more for an instrument that had signs it had been played well and frequently. Many of the Stradivarius violins still in existence stay locked in safe-deposit boxes as investments, and as a result tend to lose the tone which made them valuable in the first place.

A scratch from careless handling would be a different matter, which is why I can't sell my electric guitar. I made the mistake of carrying it around in an ex-army duffel bag, and the varnish on back of the neck got skrogged; it plays well enough, but looks too bad to sell.