I thought the general convention is to edit rather than repost (to prevent clutter, mostly), and rely on good faith that no one would mess with their own post after an agreement has been made. I assume the forum software keeps track of post changes, so old pre-edits can be looked up. If I'm really concerned, I also make sure to repeat the pricing and condition in my PMs to the buyer/seller and keep records thata way. Considering the few trouble we've had with the buy and sell section, IIRC, none of them seem to involve a poorly timed post edit.
Back when I was trying to sell my CA36's stock, I'd have felt a little silly making a new post everytime I went to a game with it. :lol:
"Item has been to one game."
"Item has been to two games."
"Item has been to three games."
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."