This could have been an interesting discussion if the accounts he is trying to sell were revealed by you. Without that, there is nothing new to discuss as you can find a lot of people trying to sell accounts in Auctions or Digital Goods sections with different low level accounts they don't care about. It can only make them lose something if the accounts they mean to sell get red tagged before they are given to the buyer. After that, if he still manages to sell it, the buyer is screwed which is a good thing too, and if he can't get it out of his hands, he is screwed for a few hundred bucks he has been harvesting all this time.