There is risk in any retail store, true, but this is not a retail transaction we are talking about. This is a private sale occurring on third party private property, whole different animal. For sake of argument, say the amp is stolen, and the store owner did not know that, and allowed the transaction to take place. Cops find out about it, now you have a nasty situation on your hands, and you'll get a reputation in the community that you may not deserve for not really doing anything wrong except trying to be a nice guy.
If you walked into a Best Buy, would they let you plug in the cell phone charger you carried into the store in your pocket, prove it works then sell it to another person right in front of them? For less than they sell them new? Or sell a used car to another person at an auto dealership's lot? As a matter of general principal, if I am a store owner, the only person making money on my property is me, that's it. That isn't snotty or anything like that. I wouldn't let my neighbor use my front lawn for a yard sale unless they paid me some cash up front.
This is a scenario that seems to come up often. If I owned a music store, why not try to make some money off it. Offer this as a service, kinda like escrow, charge a few bucks for it. Add-on sale potential is big, good chance the instrument needs new strings, fret recrown, etc. Plus it gives you first hand knowledge of what gear is trading hands in the local market and what prices are being charged. You act as an impartial third party to the transaction, can offer advice on condition and value to both parties to ensure a fair and even deal. I would think you would have to treat it like a pawn transaction though, and take photos of people to avoid stolen property sales. Might work.
