eBay rules says you need to take item back if item is not as described, and buyer pays return shipping. Politely state you will give him his money back after he returns it to you in good condition, and remind the buyer that you do document the unpacking of the item via video/photo and this evidence will be used to appeal the claim if the item turns out to be different or missing components, etc. I always record serial numbers before selling anything electronic, including photographing the internals if it's been opened before. If a part has been swapped, usually, it can be determined from an inspection by comparing to those photos.
Also, please post the buyer's eBay name. Some sellers here might want to add him to their bidder block list.
