I have been wrestling with this one for a while: long enough to pick up my bottle of cherry beer and set it down again a few times.
Some observations:
1) The person who initiated the transaction was the seller. He had already determined what the value of the gun was to him: zero. That's something we have to swallow, and it is a bit alien because nobody here places that value on that gun. But the seller did. He was willing to hand it in to the police. He didn't even do the first bit of research to find out how much the gun was worth. He was getting rid of a problem.
2) Has GotGlock been dishonest? Well, unless his employer has a rule that fair price must be paid on firearms (a certain percentage of its value according to whatever reference table is used) then according to the story so far, he has not been dishonest. If the seller asked what it was worth and GotGlock said $40, then that would be dishonest. If GotGlock told the seller that he might as well sell it for $40 because it was defective, then that would be dishonest. He made the seller an offer and the seller accepted it. That's all there is to it (if I look at this as objectively as I can, by pretending to have no interest at all in guns, which is the mindset of the seller).
3) On the issue of whether the gun was stolen or otherwise has a dubious history, I don't have any comment. I don't know how that can be checked and whether GotGlock had the opportunity or followed the proper procedures to detect that. I don't know the laws as pertain to that transaction.
4) On the issue of whether it would hurt his business if the seller subsequently found out that the gun was worth ten times what he got for it: it depends if you assume the seller tells other potential customers or not. If it only concerns the seller, I dismiss this claim that it hurts business. The seller got $40 for something that was worth $0. That's all there is to it, he has no recourse as he gave up any interest in the value of the gun when he entertained the possibility of handing it in to the police for no money at all.
5) On the issue of whether it would hurt business if a third party found out about it, well clearly it would, if the posters in this thread are a fair representation of typical customers interested in goods of that nature. They obviously hold themselves and each other to a certain gentlemanly and commendable behaviour as far as such a transaction is concerned. Unfortunately they (indeed we) forget that there is no compulsion to be a 'nice dude' and advertise to the seller what the true worth of his item is on a personal level. Whether there was such a compulsion from the point of view of the business is unknown. We would have to talk to the owner of the store and find out.
The seller wasn't raped, conned, fleeced or any other term you care to use. He got $40 for nothing. He did not expect any more. I put it to you that he would have sold it for $1 and would have felt the same sense of achievement walking out of the store. He got rid of a problem. GotGlock scored a nice revolver for $40.
Now whether this transaction is indicative of an underlying chicanery on the part of the OP with other potential transactions (buying and selling) is a matter for speculation. I need to lift this bottle a bit more before I can address that one