I reckon all that a rule like this will do is cause more people to post even more worthless drivel, more often, in order to up their post count. I couldn't care less what someone's post count is, just because someone talks alot, doesnt make what they say more legit, valuable or reliable.
What we need is a buyer feedback system like ebay/bidorbuy etc, where sellers and buyers can give feedback on sales and scammers can be wrung out of the system. New sellers would need to improve their reputation before sales can be done with confidence.
After reading Slaves post, I couldve just said - what he said.
