Just to throw in my experience.. I'm not a fan of the authentication at-all.
I purchased an IWC portofino at the end of last year through eBay (UK), which went through the authentication process. And in fairness it went very smoothly, and all of the extra bits were in the box, and so on; and the price was good. Anyway.. A week later a friend offered me the watch I'd actually wanted all along (another IWC) at a very friendly price. So I decided to put the original portofino back on eBay. It sold quickly, and off it went back to the authenticators. Then nothing for 5 days. Then I get a message to inform me that it had failed the authentication process, and was not a genuine watch.
After calling a dozen times I was eventually put through to someone on the 'authentication team', or something like that (the girl had an Irish accent), and she pulls up the notes from the watchsmith who authenticated it, and it just said "Doesn't look genuine". And there you go. In my opinion it was gen, but I'll never know. I didn't remove the caseback. I own it in gen and rep now and with an identical strap it's a close call between them. Anyway, all of that happened within the first 45 days of my purchasing it. So I returned it to the original seller for a refund. The buyer left me my first ever negative feedback, and then eBay removed it because they had originally authenticated the watch.
TL-DR - I purchased a watch through eBay which passed authentication and was delivered to me. I then sold it, and it failed authentication, with a very vague reason. They don't keep a database of watches/serial numbers, and they're inconsistent. Same watch, 2 outcomes. I would wager they don't remove case backs either, as he/she would have known fairly quickly and given incorrect movement as the reason for failing it (which i suppose is good news. I would rather they didn't take the case back off anything I'm spending money on).
I also sold a black Pelagos through eBay which had no issues and went from me to the authenticator, and on to the buyer, in 3 days, losing none of the items I'd sent. So it seems to be hit and miss, which isn't really good enough when you're dealing with more expensive watches.