In fairness to your point, I also find that Amazon reviews can no longer be trusted, and I really try to buy as little as possible from Amazon. Due to this, and other reasons, I find it quite difficult to have a good sense for whether I've bought something high quality, or if it'll be a piece of trash.

Since LLMs are trained on "random people online", why are they not of equal rank?

It depends. For home improvement projects, you can see what tools people are using. If a home repair channel is trustworthy, that may be a good start. General market research can help as well. For instance, post-China-buyout Craftsman should be avoided. For nearly anything else, it really depends. Name-brand electronics are usually safe unless they're small items (eg, power adapters, USB cords. Batteries are nearly impossible to purchase online without getting ripped off.) For clothing, I would just generally recommend only buying from thrift stores. For a lot of physical items (eg: door knobs, fuel hoses, etc.) it is actually quite difficult to purchase online. In person, we can tell a lot about quality just by touching the object with your hands. (eg: a flimsy shelf is not self-evident online, but is obvious in the hardware store.)

I suppose that's a long-winded way of saying that nearly every category of item requires its own strategy. For a brief period consumers were winning the information war and you could just go to Amazon, read the reviews, and get a superior product for cheap. We're now in a modern-but-old-fashioned situation. It's quite difficult to know if you're going to get ripped off, and you're forced to rely on more blunt heuristics. (eg: trust specific brands, buy things in person, etc.) None of these are perfect, but they are quickly becoming the best of some bad options.