For me, it's the lack of real feedback, the type of past selling transactions etc. that make this seem fake. However, there is also no warranty card present...
Honestly though, who actually sells a fake IWC on ebay? eBay is near impossible to register to these days without handing over at least some of your contact information and anyone who buys an IWC from you, if fake, no matter HOW good the fake is, will eventually find out. It's just a matter of how long till they send it to Schaffhausen or to the London office etc. to get it serviced and BAM, "we regret to inform you that your watch is counterfeit blah blah". Hell, even a Concepta or other Genestein will be busted if they send it to get serviced. Having paid over 3 grand for the watch the buyer, eBay and paypal will hunt you to the ends of the Earth and you'll probably end up in-front of a court, because it's pretty big-time fraud conning someone into buying a multiple-thousand pound watch.
For that latter reason I like to think no-one is stupid enough to try this and subsequently I'm going to say it's genuine...