I work for a non-profit that is largely oriented around the acquisition and resale of salvaged building materials. We have criteria for goods we accept as donations, and we have a "Dollar Door" section for doors that don't meet our criteria, but somehow slipped in. Now that they're here, thought, we'd rather find them homes for a dollar than just recycle or landfill them.
So yesterday, a co-worker approached me for advice on a situation. A woman, on her very first visit to our store, had found, in the Dollar Door area, a set of old, beautiful, divided-light french doors. They had some issues (a broken pane, serious dog scratches, etc.), but clearly weren't really dollar doors. In the condition, they were in, I told her, I would normally sell the pair for $65. I asked her if she would be willing to pay $5 for each door instead of one, and let me strip the ball hinges (she was making a headboard out of them and so didn't need the hinges), and of course she agreed and left happy as a clam.
I don't think you've done anything wrong, morally or legally. If you want the gun, you should go buy it and feel done with the matter. If you personally feel some moral ambiguity on the matter, which I guess you do since you're posting this query, then I think you should go to the manager and be totally candid and upfront with them. Explain that you would really like to have the gun at the listed price and see whether he's fine to leave it at that or if he has a counter-offer that is fair to you and them. In the current economy, I bet that if you made it clear to the manager after bringing the issue to their attention that if the gun cost $399, then you wouldn't be buying it, you'll get a great price.
Josh