The two methods:
1) Forfeit Tesla account to buyer (may not be possible if seller owns a Tesla car). This is the easiest IMO.
2) Change the registration info and delivery to buyer's (also requires driver's license upload). See below.
For two - since Tesla doesn't support it as it is a loophole and who knows if Tesla will crackdown on it in the future.
1) Get SA (or customer service) to reset the registration info and delivery location on order. Might not be possible to change delivery location if already assigned VIN with delivery appointment.
2) Enter buyer's new registration info and delivery location. Driver's license will need to be uploaded.
3) Buyer pays for car (order still tied to original seller's account).
4) Seller transfers cars into buyer's Tesla account.
Requires some trust as the new buyer is paying for a car tied to the seller's Tesla account.