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.