Clarify.
Sounds like you have it bound to your account (since you've been using it), now you sold it and the new owner is saying it's bound to another account.
I.e. you have to go into the DJI Fly app and remove the drone from your account so...
Already asked D-H support this question (over 2 years ago) and the answer was "...D-H hacks are installed to the drone - not the controller..."
Have you tried using DJI Assistant 2? With the RC-1A/1B it was (and still is) possible to reflash and...
Sounds like you're looking at asking prices and not "sold for" prices, common mistake on ebay.
There are always people asking for unreasonably high prices, and that'll simply never sell until they adjust to the market.
There are no "one size fits all", if that's what you want then best stick with the defaults and get used to them.
Otherwise it'll depend on your own feelings and what you're trying to do, so experiment.