The reason Alibaba will take over from Amazon and Ebay is simple.
Have you ever ordered from China before? Because your comments overlook a few things.
1. Shipping. I have never received anything that's been shipped from China in less 3 weeks, and it's usually been closer to 5 or more. (Coincidentally, just before finishing my final comments below, a small package of keychain LED lights arrived which I ordered 4 weeks and 6 days ago.) Sometimes that's ok if it's a cheap gadget I don't need in a timely fashion (keychain LED lights being a perfect example), but for most things I'm willing to pay a few extra bucks to have something within a few days instead of a few weeks, especially when you factor in point #2.
2. Trust in the seller. If you have ordered things directly from China, as I've done through eBay, DealExtreme and others, you know that often what arrives after 5 weeks can be of shockingly poor quality and/or not what you paid for. For example, I recently ordered some US to Euro AC wall plug adapters. They were assembled so ridiculously poorly that I had to disassemble and reassemble each one for them to be decently functional (still scratched all to hell). In another recent instance, a seller got into a drawn out argument with me when I left them a bad review for shipping a terrible quality product: a microUSB cable that wouldn't charge any smartphone and which didn't match the one pictured, though it was close enough that I'm sure they were hoping I wouldn't notice. They were of the opinion that because they refunded my money I shouldn't leave them negative feedback, and whined like a 4-year old about how damaging it would be to their business. Granted, you can have such issues in any virtual marketplace, which brings us to point #3.
3. Trust in the marketplace owner. I trust that if something goes wrong with an Amazon Marketplace transaction, Amazon will go to basic lengths to make sure I'm not getting screwed over. The same goes for eBay to a lesser extent, but still more than Alibaba. I still haven't ordered anything through AliExpress. Why? Because I'm taking enough of a gamble already ordering from Chinese sellers on eBay, who is known for always siding with the buyer in disputes.
As someone who occasionally sells online, I am glad that Alibaba is getting some attention and creating some competition for eBay (and with it, Paypal). But to suggest that Alibaba will put the likes of Amazon out of business seems rather short-sighted -- or at least premature. You may have a point with eBay -- at least giving them a run for their money if Alibaba opens up to US sellers seeking refuge from eBay's excessive selling fees. If they're smart, this is exactly what they'll do with all this IPO money.