Ok we ban account selling, it can't be enforced, and then it becomes more dangerous because people think it can't happen.
It can be enforced. Anytime someone proves that the account belonged to him earlier restore it. There will be those sellers who will try this, and after a few cases all the buyers will be wary. Don't give any protection to buyers, if you find an escrow doing this ban him too.
Not only are you helping hidden selling of accounts, you're actively helping it by making it open and allowing escrow for it.
In addition to the enforcement issue: From time to time people have asked me whether they could sell their accounts or use them as collateral because they have an emergency need for money or have come on hard times.
In that case they can do so openly. There is no need to make it hidden.