If the buyer is out of state, you just send it to the dealer of their choice. You don't need to unregister it here (this is why the registry is not particularly useful).
Make sure you get a copy of the FFL certificate of the dealer, of course, so you can confirm they're legit:
https://www.atfonline.gov/fflezcheck/
Cashiers Check, or Money Order, or if you allow Visa, MC type payment, but unless you are a store it is understandable you cannot.OK thanks.- so what is the best way to receive funds?