First it would make it extremely easy to scam account buyers who do not think to ask for a signed message and would make any buyer vulnerable to having their account taken back in the event they lose such signed message.
Good, let them be scammed. The only reason account trading is allowed is because they can't be stopped. If the rules favour original owners then account trading will reduce drastically.
Go to the Digital Goods section, you will find a few newbie sellers selling a lot of accounts. Why are they hiding? Simple as they know they can sell and leave leaving the buyer stranded. If the accounts are reset to original owners then the buyers will go away.