But they're not marks of operational quality. A UL mark on a microwave oven doesn't tell me that it's a high-quality item that will cook my food better than an unmarked item, or that it is something that is easy to use or will stand up to the test of time.

It just means that it's safe to use, according to whatever tests were performed on it. The UL (or other NRTL) mark means that it's extremely unlikely to give anyone an electrical shock or burn a house down, even as the device fails.

As another example: A power supply brick for some manner of widget. The UL mark doesn't mean that it will last for years and years, or that it will power whatever in-spec widget it is thrown at.

The mark does mean that when I short the output terminals together that it won't catch on fire. That's good! But it does not mean that the power supply will ever work again.

An item can fail both absolutely and permanently, and as long as it fails safely then that failure isn't held against it. It can still carry the mark.

The UL mark is valuable, as are the other test lab marks. However, while an ethical storefront would require it, Amazon does not. I've bought plenty of things that are not listed, and if I want safe products I generally have to include "UL" or "ETL" in my search. Even that doesn't always work, I've seen products on there claiming a listing in the title but photo shopping the place where the UL logo ought to be off of their product images.