I've had good luck with Amazon personally, but I do avoid entire categories of stuff (anything with an Apple logo, to start with) and largely stick to single-seller things anyway by the nature of what I buy there.

If they did start sending me junk I'd return it, and complain loudly. So far I've only had to engage their customer service about a few delivery-related screwups, but they've been extremely helpful for that stuff, so I'm curious what their threshold is for actually addressing stuff like this.