Can I sell my car registered in Rayong while living in Bangkok, or do I need to go back to Rayong for paperwork?

No you don’t need to do that. The new owner will have to register it with the documents you give him or her. If that is in a different area to where the car is currently registered, then they will be sent back to the office before the re-registration could be completed. It usually takes a couple of weeks depending upon which area. If the re-registration is being done in the area that the car is currently registered then it can all be done on the same day.

Any MOT station where you get your tax can do it for you.. usually takes a couple of weeks. Car doesn't need to go Rayong.