I see the problem Lud is referring to now. If you need to edit, you can't currently do it with a reply, because a reply is a bump. Replies are limited to 30 days to limit bumps, and therefore edits in a reply would be limited by the same time period. Lud is worried that someone will not post correctly, need to edit, and now for 30 days he can't.
I guess one possible solution is to force sellers to place one ad per item, and include the asking price in the title (which never seems to change even if you try to edit it later on).