The gentleman I saw that day did not happen to have anything I was interested in. But, I could not help to overhear the conversation. I think the gentle man accepted the trade-in value offered for two of his guns, and decided to keep the third. Hearing this just happened to get me thinking about the situation.
I also think it would be rude to interrupt negotiations. But, I would likely talk to the clerk and gun owner if I ever saw something I was interested in. If the shop ended up buying from the seller, I would imagine the shop would cut you a good deal to make a quick profit, instead of letting the gun sit in the display case.
In my opinion, it would be also be rude to offer only a few dollars more than the gun shop. Let us say the shop is offering $225,and the owner wants $400. Offering $250 would likely piss off everyone. However, if the shop and owner could not reach an agreement, I do not think the shop should be too mad if you offered the seller $350. At least in this hypothetical situation.
I like the idea of giving the shop an opportunity to do a transfer for a fee. Or, perhaps simply offering the shop a finders fee would suffice.
Then again, I am a newbie to the world of guns and I will likely find many things to ponder. I thank all of you for sharing your thoughts.