I think that a good solution would be to just not allow threads for account sales. Bitcointalk already doesn't allow sales thread for illegal items. Although account sales will still go on, offsite, the inability to advertise on this forum will hamper the marketing efforts even further. After all, the target market is people who already use this forum or wish to use this forum. (Most probably to participate in a signature campaign.)
   I'm not certain that this forum should take the extra step of banning accounts suspected of being bought/sold. If it did, I would expect the level of certainty that the account is indeed bought/sold be rather high. I would hate to see an account banned erroneously.