Useful for determining when an account has been sold too.
Yes , but when an account has changed the pass it doesn't mean that it was sold , it is also possible that it been hacked.
It is also possible that the owner has only decided to change his password.
Wouldn't you be a bit more cautious trading with an account that had this warning message? It lets you know that the account was possibly hacked or sold, allowing you to be wary of any previous trust/reputation it might have. That seems to be the use of the warning message to me.