I am selling an FAA STC to a company outside the United States, what do I need to do?
An FAA STC can only be transferred (and reissued) to a person or company outside the U.S., if there is a bilateral agreement in place with that country and the bilateral agreement includes provisions for transferring STCs. The bilateral agreement provides for transferring the associated State of Design responsibilities between the FAA and the other civil aviation authority. You should contact your local FAA Aircraft Certification Office (ACO) for additional guidance well before finalizing your business negotiations.

Bilateral agreements policies & guidance