No, you're wrong, actually. The password is the password for the backup file. You will still have the backup file and know the password you entered for it. This effectively allows you to always use that backup file, regardless of whether someone else also used it or even created a new backup and set another password. It would effectively come down to a race of who first sends the money away from that wallet, since you will both have access to it.
If the person who bought the wallet added even more bytes to his wallet in the meantime, you would actually have control over those as well.
I'm definitely not saying you would do so, but it's only fair to state the obvious risks involved generally if someone buys a wallet backup from someone else. Also, please note that the wallet seed does not restore private assets, tokens, smart contracts or paired devices. For that, a full backup is needed. So only selling the wallet seed would result in the buyer not getting the funds locked on the smart contract.
There's no "safe" way to transfer a smart contract, since it requires the full backup file and that can be kept by the selling party. (again not implying that you would, only speaking generally)
RE: I NEED HELP! Can I sell my Byteball Wallet? [Best Discount Price]