First of all, HN is full of entrepreneurs and wannabe entrepreneurs. A storefront that can't sell anything that doesn't have a good reputation outside the storefront is bad for small businesses, because nobody is willing to try anything new. A major part of the value proposition of a storefront is that the owners will vet the products they sell for a minimum standard of quality: it's not necessarily great, because greatness is subjective, but the food shouldn't poison you[1] and the AC/DC power converters shouldn't catch fire.
The second problem is that sellers will pass off counterfeit goods as being from major manufacturers when they aren't. I know I'm falling for selection bias, since I've probably bought counterfeit goods without knowing it, but that's not even really the point. The point is that I've bought stuff on Amazon that just didn't work, sold in packaging that's identical to stuff I've bought before that did work. I knew they were counterfeit and not just duds because the actual product didn't have the branding on it that genuine ones always have.
[1]: Well, okay, it might poison you if you have a special allergy. But anything with the common allergens like gluten or lactose will have labels.