Thanks guys, your insight is appreciated. My wife is sick of me telling her this story and picking her brain for investigative insight. A couple of clarifications:
-- I typed the original post in a narrative form from memory. It's something the lawyer I consulted asked me to do, and I've been meaning to do for a while-- I just had a chance to sit down an plow through it yesterday. I recalled my buyer's trigger that it was a replica being information he received when he took it in for a service. Going back through the emails (I've been going back and forth between all parties involved over the past 7 months) to get details for questions that were being asked, I read (and then remembered) it was Tourneau that spotted the issue when my buyer attempted a trade-- it wasn't a service issue. Nothing nefarious in this part.
-- I agree that the simplest answer is usually the right one-- Occam's Razor. In this case, I've been corresponding with my buyer and seller for a while now, and both seem like stand-up guys. I do not think either of them is the (knowing) bad guy in this situation. The likely culprit in my mind is the Pawn Shop (or whoever sold the watch to them). And even if I didn't know enough to spot it as a fake when I received it, the fact that the crown came off gives me a lot of pause in retrospect. However, any doubts that might have introduced were erased when the watch was serviced and returned to me by a Rolex AD.
-- If the crown thing was just a fluke, and the pawn shop/eBay transaction was completely legitimate, then I sold a 100% authentic watch to my buyer. The watch was not opened again until the attempt by my buyer to do a trade-up at his local Tourneau. Based on my conversations with my buyer and research on him, I believe this to be true. Would it be even remotely in the realm of possibility that Tourneau would swap out the movement? It just doesn't seem plausible.
-- Based on everything I know, either (a) I was sold a very high-end replica, and the third party service center my AD used just missed it and slapped an OEM crown on a knock-off movment, or (b) the service center was the culprit and THEY swapped out the movment, or (c) my buyer facilitated a movement swap to try to swindle me, or (d) Tourneau has a side-business moving Tudor ETA movements on the side.
Are there any other possibilities I should be tracking down?
I was extremely uncomfortable with the potential thought that I had sold someone a fake watch, so I erred on the side of refunding him. If I'm out $2000, so be it-- that's better to me than sticking it to someone that I'm 95% sure is innocent. My ideal situation would be to identify it as a fake from the get-go via the serial number, as that would clear up all doubt as I have that number accounted for from the time I received it, to the service, to now. Is there any way to do that? If my serial number isn't a legit number, that would do it. If my serial number is a "known" replica serial that turns up frequently, that would also do it. And if my serial number is legit, but was sold to Bob in Topeka, KS and has been serviced in that last couple of years at his local AD, again-- that would do it. My attempt to get this from Rolex was unsuccessful. Any sources/contacts you would recommend? Again, I appreciate that help and insight.