A great question. I only know a little about Dunhills, but I'm constantly watching for one in my birth year, 1961. It can be difficult. Sometimes sellers don't know they have a fake. However, I suspect there are more replacement tenons than fakes out there, but you still have to do your homework. Stamping and stem design are critical elements. There is way too many nuances for a newbie to learn and be able to identify from an ebay pix. When I find a '61, I'll be sure to send it to a few trusted Dunhill experts for confirmation. Get to know those guys on the board. There are a ton of good Dunhill webpages out there, study them if you really want to gain some knowledge. I'd also recommend the "Streets of London" social group over on the SmokersForums.uk, those guys know their British pipes. Chat up the Dunhill guys here as well, of course. We have several serious collectors.
As mentioned, you are pretty safe buying from someone like coopersark, secondhandsmoker (he has a wonderful '61 on now!) or pipestud on Ebay. Their reputations and pipe knowledge is sterling and they are well respected in the pipe world. Mr Can is also someone who has a lot of Dunhills. Be warned, their pipes never go cheap.
As in every collector purchase, if you get a "deal" on a Dunhill, it might not really be a deal!