I have to wonder why the buyer didn't think to ask you why you decided to take a pass on a gun he thought was such a steal that he bought it at "first touch". Frankly, unless you're asked for your opinion and/or expertise, the transaction between store and the other man was none of your business.
While it's difficult to see someone make a misinformed mistake, the ethical problem was not yours. Perhaps the store salesman didn't have an ethical problem either. He did, as you stated, open the gun up for your inspection which is how you knew it was a salt gun in the first place. The fact that he didn't identify the actual reason for the corrosion correctly is inconsequential...the damage was not hidden from those who asked.
Bottom line - "stupid is as stupid does"
Forrest Gump
