The insurance provider needs to issue you a certificate stating your covered in Thailand for covid. If whoever your credit card uses to provide insurance will do that then you can use them.
So 2 things, if he's bought the bike from you the greenbook and bill of sale are his.
2nd, If the seller of a bike or car is a foreigner, the NEED to accompany the buyer to the DLT with a certificate of residency or work permit AND their valid passport, otherwise the DLT won't transfer ownership. (varies slightly from office to office)
3rd. Most people don't transfer ownership of a bike when they sell it. My bike is still registered in the last persons name I bought it from, it's just easier.