hi there,
i in no way get hold of a users private keys. if someone wanted a name, then i transfer full ownership of the name to their wallet. i only ever know the wallet address that I have to transfer ownership to, that is all, and theres no way i can find out private keys or seed phrase at all, ever.
I hope this allays your fears.
Please explain how this is done and how you can assure the buyer that their private keys are safe and not exposed to anyone except them. That will be great starting point and will probably get you more sales also.