Lets refer to the Google account you're securing as "OGE". (original e-mail)
For this, you will need a safe and personal Gmail as a "Parent" or "Supervisor" for control over the OGE.
1. Have the seller check if there are any Brand accounts linked to the OGE (brandaccounts in google settings). If yes, ask the seller to delete them.
2. Then, ask the seller to check if there are any payment profiles on the OGE (payments in google settings).
If yes, make sure the country matches with your personal Gmail's country.
If not, ask him to delete the current payment profile and make a new one with the same country/region as your personal Gmail.
If there are no payment profiles on both the OGE and your personal then you don't need to do anything.
The main thing to remember in step 2 is that the country/region criteria should match on both the OGE and your personal Gmail.
3. Change the birthdate of the OGE to under 13 years old (birthday in google settings). Google will then prompt you to add your parent for supervising the account. Use your personal Gmail here and make sure to select "Yes, ask me every time" for the sign-in controls. Enabling this will require anyone trying to log into the OGE to also login your personal Gmail.
Also remember to use your own recovery info on the OGE and keep 2FA enabled.
This will now add the OGE to your Google Family with your personal account as the Supervisor.
Lmk any questions, i'll answer them with what i can.