This is my experience as well. the amazon site seems to make it look like I'm buying through amazon, though it's actually sold by 'some company in the small print'
I agree with the OP that 15% is pretty high for selling commodity stuff, but amazon seems, ah, sub optimal for lead generation because they make it look very similar to just buying from amazon. Like you, I've never gone from buying through amazon to buying direct from a merchant.
I think in some cases, the 15% might work out. As far as I can tell, when you buy in small lots you can often get a better deal than larger, more "reliable" lots, just 'cause someone further up the chain from you took a loss, (like someone bought a bunch, then went out of business and liquidated the lot at below-cost prices.) or because corruption is such a large problem (the larger the corporation, the more kickbacks are expected, as far as I can tell. If I own the company, kickbacks aren't going to get me to choose an overall more expensive vendor, it's my money, either way.)
either way, from what I've seen, often smaller lots from smaller resellers can be had cheaper than larger lots from more reputable resellers, even after you amortise out fraud/misrepresentation losses, so it's possible that the guy who sold me the cheap ESD wristbands on amazon really has more margin on the product than amazon direct would have.