I don't know about the no bump unless lowering price. I know what you're getting at as the threads get out of control with some people bumping incessantly, but at the same time, the forum moves so quick at certain times of the year that FS posts get buried rather quickly.
I like the other ideas though. I would also recommend a stickied thread for those who had good/bad experiences buying on the forum. That way buyers/sellers can search that thread and get some comfort for who they're dealing with.
I know what you're saying about bumping to keep the FS threads on Page 1 of the "active topics". but to me if you're searching for gear you'll head over to that section of the forum. Bumping sort of creates an unfair advantage for the guy that's willing to go on daily and bump his thread. I'm on a hunting/shooting forum and they have very strict bump rules because it got out of control.
Rule #4 should be that you have to have at least X number of posts before starting a for sale thread. At least an intro post on the intro thread.