Thank you for your replay that was helpfull so much.You should just deliver the gmail account and password so buyer can have all access with it.
If you don't want to give all access to google services, open a new gmail account then move your google cloud billing administrator role to this new gmail account (Google Cloud>Billing Page). Then give him the new gmail.
(You can't sell something to users in this forum if you don't have a sales thread.)