I think it comes down to honor mostly. And mostly, people seem to have less of it.
I go by time stamp. And once I agree to a deal - whether buyer or seller - I stick with it. I've agreed to sell an item I had listed for $225 for $200, then a bit later had someone offer the full price. I agreed to the first, I'll stick with it. I've had people pissed because they claimed to be the first "I'll take it" when I'd already agreed to a deal. I've also had the seller go with a higher bidder after they'd agreed to my offer.
This^^^
I work in the same office with another OSA member. In the last month or so we have both been jerked around by sellers. I sent a PM and then posted that I would take the deal. I didn't hear back from the seller for a day or so. Then he said that I was second in line behind a person that had PM'ed. No biggy, I told him that if the first buyer fell though, I would still take it and he agreed. A couple of days later he PM'ed while I was at work, and said that the first buyer fell through. I PM'ed back to arrange a meeting, when I got home that evening, and when I went to the ad, he had posted it sold to someone else.
My co-worker arranged a deal. Made an agreement to meet up, and when he got in from work, he found that the buyer had sold the item.
It comes down to your word. If you say it, do it.
This too^^^