Do you have any examples of this? It seems that most scams are attempted by low level accounts and accounts that are unlikely to have been purchased.
If you can't get a signed message for any reason then it would probably be best to have that person send first or to use an escrow service to protect your money
I posted the case above:
https://bitcointalk.org/index.php?topic=936429.0 (where the accused had sold his green trust account which was later used to scam a well known scammer).
Krishatnet's account was sold and it had green trust. The person who traded with him trusted him because of the trust rating. But unfortunately, the person who was scammed turned out to be a scammer himself so his case wasn't entertained. There can be more cases as well but they might not have been addressed.
What makes you think the account was sold? I see that you had posted in that thread saying you know him from elsewhere, but unless he gave some kind of proof then him claiming it was sold would likely just be him trying to protect his reputation.
The account was likely worth more then 1.3
BTC (the amount scammed) so the scam in question was not economical for the owner of the account. Sure the victim is out $325, however account owners would have incentives to not scam with such accounts.
Also banning the sale of accounts would likely cause the prices of accounts to decline (there would be an additional risk the account get banned if it is discovered to be sold) so the above incentive not to scam would be removed (or lessened)