I think you'd need to play a different game altogether if you wanted to carve a new niche. What that looks like, I'm not sure. Maybe a shopping site that builds carts across various retailers and orders from the least expensive?
As Amazon gets into more sectors, competition will increase. I don't think that will kill Amazon, but a big breach or other bad press could be enough to get people to move to alternatives, especially if those alternatives have become more competitive.
I don't think startups can compete until larger businesses threaten Amazon (Costco, Wal-Mart, CVS, Target, etc).
I tried ordering angled cutting pliers for cutting excess legs off of my PCB's when soldering, and other than Hakko which was an add-on item so I couldn't buy it unless I wanted 3 of them, most other brands were cheap Chinese trash.
I've had 34/35 items arrive from Aliexpress in the last month from Cambodia, China, and Russia. One was mis-delivered by USPS in USA. Amazon can't deliver correctly to the Whole Foods 1/4 mile from their HQ.
Sounds like pcpartpicker. Which is, of course, only relevant if you're trying to buy parts for a pc. Great service though (not affiliated in any way, just a happy user).