I understand what you're saying, but I like being able to go back and being able to update and edit the original thread. Being able to add or delete parts as they are sold helps when you have a continuous running FS thread, instead of creating a new one every time.
For the most part, we have a good user base and regulate ourselves pretty well.
If you "quote" them then they can't change the price
The buyer can edit the quote just as easy too to fit their needs.