In a well-known incident in 2016, Apple sued a company making fake "Apple" power adapters, and in the suit alleged that 90% of the "Apple" products on Amazon were fake.

That such adapters are literally dangerous is clearly documented in Ken Shirrif's teardown:

http://www.righto.com/2014/05/a-look-inside-ipad-chargers-pr...

http://www.patentlyapple.com/patently-apple/2016/10/apple-su...

http://www.businessinsider.com/majority-apple-accessories-am...

https://lifehacker.com/how-to-spot-a-counterfeit-apple-light...

Surprised nobody else mentioned the possibly counterfeit eclipse glasses:

https://arstechnica.com/tech-policy/2017/08/lawsuit-amazon-s...

Amazon did in fact try to avoid shipping fake glasses, but some may have gone out.

Hoverboards were a big one a couple years ago - burned a few houses down and killed at least one kid. The vast majority of brands of hoverboards sold on Amazon, as far as I know, were private labels from the same handful of Chinese factories.

I would also especially worry about cosmetics and baby products/toys and I hope nobody's buying power tools from Amazon. There was an article here on hacker news a couple weeks ago that mentioned a lady who was (partially?) blinded by an unsafe dog leash she bought on Amazon.

Counterfeit nutritional supplements. No way to tell what's really in them.

Edited for clarity

That's true in grocery stores too though

What grocery store let random people stock their shelves?

There is no oversight of supplements (at least in the US). You just have to trust the brand, because legally there's nothing stopping a supplement manufacturer from using entirely different ingredients from what is claimed.

Yes, but at least you can trust the brand or not at most brick and mortar stores.

On Amazon, you can buy your trusted brand and not actually be getting that brand.

Granted, this happens at brick and mortar on occasion, but they generally have a relationship with most of their suppliers and it would be unwise to allow counterfeits through.

Electronics which have a UL sticker but are nowhere near UL standards of design. It probably won't burn your house down, but it's a lot more likely than with a UL-approvable design.

Baby formula is a classic one. Electronics with flammable batteries.

Cosmetics. Supplements. Medical devices. Bike helmets. Just to start the list.