Everyone know that accept collateral as an account is equal to trading account. I can say it's legal way to sell an account. Because I have seen previously if you tag them then you will receive retaliatory tag. And then someone from DT1 will put him on DT2. Then ther will start another story what happened with The Pharmacist previously. And I believe this is not a good practice for DT member.
That's why no one want to tag this kind of case. But I strongly agree with tag sold account even avoid tag to lender.