While all marketplaces are caveat emptor, on Amazon I am up against Amazon with all its staff, scalable computing hardware, and algorithms out to maximize the amount I pay. I may not trust a particular seller on eBay, but that's better than not being able to trust Amazon.
I know, it's hard to believe. But there are definitely quite a few niche pieces of software (and technology) which Amazon doesn't supply.
Outside of that, deals and things help too. I mean, why do people buy games on Steam or the likes? Because there's always a sale on and it's got a better selection than Amazon for that market.
The site itself? Fraud. Awful, useless search/filter results. Fraud. Did I mention fraud?
And this question reads a bit like an ad.