Thanks for the replies - I've just finished on the phone to ebay and I'm actually pretty astounded!

for context the buyer raised the case and gave the following detail:

""Hello there, I have now looked into the detail at the back of the watch I suspect this watch to be a fake. The engraving on the back of the watch in the photo is a lot better than this. The weight of the watch is also too light."

*Interesting that in the case detail they never mentioned serial number, just in their direct message to me*

My assumption was that the watch goes back to ebay to investigate his claims and if valid, they come after me for a refund, and tbh I'd also expect to be banned if I was caught selling a fake watch. But no - all they do is check the watch is still the same as when he bought it e.g. he hasn't damaged it or kept the box etc and accept the return. 

I had to check this 3 times with her...so even if ebay agree the watch is as described and authentic, the return is still valid? Yes. the only time they don't accept is if it's outside of 30 days or the reason is simply the buyer no longer wants it. She even made it sound like it's fine because I get the watch back and can sell it again?!

Again I questioned this "so If I buy something on ebay and want to return it, I should make a false claim rather than be honest that I changed my mind?" and he said that even if they suspect the customer is giving "fake reasons" they still accept the return so long as the goods are in the same condition. She even gave the example that if the watch is black and the buyer says it's pink, when clearly it's black, they would still accept the return regardless if you're a private seller. 

In future she said I should leave the funds in my ebay account for 30 days as a return request could always be accepted in that time. 

I still can't quite believe that's their policy, but I was speaking to a Brit so it's not like anything got lost in translation.