Below are two coins that I sold this summer on eBay and both were declared fake by the buyers. I am not infallible and do make mistakes (just ask my wife ;D), but in both these cases I was sure that the coins were genuine. I asked both people why they thought their coin was fake, but received no reply back from either. Interestingly, both coins (see below) were the same RIC number-- RIC VII Ticinum 167.
I see no reason at all to think they might be fake either - it's a pretty common coin, maybe they found a better deal!
Both of them had little feedback on eBay and I suspect both had little or no experience with ancient coins. I refunded them, told them to keep the coin, and then blocked both from bidding on any of my future auctions.
That makes sense - perhaps they thought they were in too good condition to be genuine ...
The second buyer (who is a university lecturer in literature) said that an "expert" in numismatics told him that the coin was not genuine. I asked him why it was not genuine and who this "expert" was...I never heard back from him.