Various schemes have been suggested before and have met with pretty universal condemnation from the members here
@Robert Docherty.
We operate our own Classified Sales Section here which is accessible once you have made your Post Count up to in excess of 100 Posts. The reasons that this was put in place was to combat the problems that you have already high lighted when speaking about various fb groups. Since the implementation of this rule, problems which were legion have all but disappeared. This works well for all of us, whether Buying, Selling, Trading, or simply offering items as Giveaways. By the time a member has amassed a sensible post count, we all have a pretty good idea about them and are better placed to make our own judgement.
We also operate our own version of a Feedback System which further helps with decisions about with whom you may be potentially dealing.
Unlike some other facilities, our Classified Sections are constantly monitored and any errors or slightly questionable listings are dealt with pretty rapidly.
In short, we are a very successful forum, we have nothing but our members best interests at heart, so you'll understand when someone comes along and makes suggestions such as your's that we might get slightly nervous about motives etc?
Sorry if that seems harsh, but that's how we roll.