If an API is returned by a customer for non-quality reasons, example, inventory levels, etc., and the customer sends documentation stating that the API was handled to GMP, could the API be resold without reprocessing?

This would be addressed by the section in Q7A dealing with returned materials where it says that
if there are any doubts regarding the quality of the returned API, for example, if you notice
evidence of tampering with the returned material or say the material originally had seals, factory
seals on the containers, and these have been broken, which may suggest potential tampering of
that material, than now you have reasonable doubts regarding the quality of that material, in
which case I would say that you should probably not resell it as it and it should be reprocessed,
reworked or destroyed.