Hi, don't fully understand why you didn't do a full reset via settings (general settings /factory reset), but you are where you are.
Giving the buyer your Account detail is a fairly big risk tbh and I think that's something you need to decide for yourself.
If you give access for a limited time, and at a specific time too - then this might be the least risky option.
change password
tell buyer they can access for 15 mins from time A to B.
after time has elapsed change all password and check nobody has messed with the recovery numbers/emails.
then I'd keep a close eye on everything for a while.
or, you ask them to return the tablet, you reset and resend.
sorry can't think of a better option.