It's actually not about if it is allowed to purchase an account but the fact that this guy's go for higher ranking account to ask if they can purchase it shows that they want to escape the hard work of growing and also may have negative intentions like using for example the OP's reputation to scam gullible victims

But even if selling of account is allowed maybe there should be a notification on the profile of the account that it has changed hands from the previous owner to a different person.