There are certain watches that are almost impossible to fake to anywhere near the standard of the real thing without actually spending a huge amount of money... Most Breitling models for instance can not easily be faked well. I would say that if an Ebay seller is UK based, i.e. both they and the watch are in the UK , then the chances of it being fake if it looks real are very small. Private sellers can always be checked out fairly easily by getting the story of the watch from them , origin , etc etc ... If someone deliberately sells a fake on eBay they are committing a criminal offence in the UK... so one of the easiest ways to feel safe is to ask for written confirmation that the seller knows the watch is 100% genuine and how they know that. If they can not do that , walk away. I once looked seriously at a watch in eBay and asked just that , but got "to the best of my knowledge" in reply. Which just is not good enough for me , so I pulled interest immediately and bought elsewhere. Also , with so many ADs having great pre-owned watches these days are fair prices , its not worth the risk if there is only a slight doubt. Finally one really good piece of advice to tell a genuine watch from a rare but very good fake if a VERY high amount of money is at stake ,,,,, ask the seller to send you a video of them weighing the watch case only with grams accuracy. Then check out the data from manufacturing specs. Fakes are almost never the correct weight.