When I post a gun for sale here, I generally post it on Armlist or gunbroker, too. I clearly state, if I have it up for auction, that I will entertain offers until the first bid post on gun broker. If I get an offer, I suspend the auction. If I get a bid, I mark the post sold pending funds. If I have it here and Armslist or another forum, I reply in order received, PM, E-Mail or Forum post don't matter, and I sell to the first person who offers my asking price. Unless they turn out to be someone I don't want to have my gun.
I see a lot of post here that state, "First I'll take it, takes it." I don't like to go that route myself, buying or selling. So I don't put that in my post.
I want to look a fella in the eyes and shake their hand before I decide to make a deal, buying or selling. If they don't like the item or I don't like the buyer we are both free to walk. I think that is fair.
The gun I recently sold, which I believe prompted this discussion, went to the first person who responded. He paid what I was asking, after inspecting the gun, and showing me his LE credentials. I walked away happy and I'm pretty sure he did too.
On an auction site, I've agreed to sell the the highest bidder when I open the auction. I'm ok with that because it goes through an FFL. I would never agree to sell, to any joe with cash, on a gun board. Even the fine upstanding folks here