As someone who works at one of the rare noncorrupt Best Buys on this planet, I can tell you that the service plan is transferable. Once you know who's going to purchase it, take the service plan brochure into any Best Buy, and ask them to update the service plan information witht he new owners name, address and phone number.
Alternativly just make sure you give the the brochure to the new owner. If you don't have the brochure, just let the new owner know if they ever need to use the Service plan, to just use the phone number that you gave when you purchased it.