I don't think *most* people will take the time/effort to find a new place to sell, then go through the registration process...to them it's probably not worth the hassle.
Ebay has almost become a lifestyle to many...once you get addicted, you just keep selling and selling...when you run out of things to sell, you start selling other things (you never planned on selling). I think many of the auctions we all loved were from this very reason.
I bet most of the parts that *would have been* sold on ebay, will now just find their way to local gun shows and such...with a small number of those making onto gunbroker or the like.