I don't think it makes sense to treat an agreed-on price as set in stone when you're talking about a musical instrument that the buyer hasn't seen or played. I think the seller is honor-bound to stick to that price if the buyer decides to go through with the deal. But the buyer is entitled to renegotiate, or back out entirely, after checking out the instrument. It's an asymmetrical situation because the parties start out with different levels of information.
It would be different if the guy said "Oh, there's a paint chip here that wasn't listed in the ad. Mind knocking off a few bucks since I have to get that repaired?"
This was not one of those situations.
