I'm taking the same position facebook (ugh) does, which allows edits. One bonus of this particular forum software is that it shows the date/time of the last edit (and maybe one can see the entire history). Screenshotting works well if people need to keep a snapshot for archive/legal reasons, but I suspect shenanigans like changing prices (or wording) after the fact will be rare, and met with disdain by the members if it becomes common by a given user.