Unlimited editing can cause confusion, lead to accusations of inconsistency or downright deception,
and put a large burden on moderators trying to resolve disagreements.
Any FS ad should be left untouched, except for substantive corrections during the normal edit window.
If the audit tool can be viewable to all members this shouldn't be much of an issue (still checking to see if this is possible as currently only admins/moderators can view the edit trail). Once a classified is edited there would be a notice on the ad stating it had been edited -and ability for the buyer to expand a view and see the previous edit history (exactly what was edited). The reason this could be beneficial is if someone lists 10 items in one ad, they can edit the original ad over several weeks placing "sold" next to the individual items in the original thread. Or maybe they had originally listed a camera and lens combo, but 2 weeks later sold only the lens. If it turns out edit history shows misleading edits their access to the classifieds would be removed so I doubt they would try anything sketchy.