I have no idea why they would "recover" the account because if they are doing the upgrade by themselves, they already have the password. Yes, they can change the password or register a social networking link but you can easily recover it back or delete the link.
But to be on the safe side, if the seller is not trusted then do not ever give them the password to your account.
no the seller can only charge back which puts your account to negative membership and basically locked.
choose who you buy from to be careful
good luck!