One or two orders of magnitude? Isn't that ten-times or a hundred-times more expensive? What product could possibly be marked up by 100x?

You can get 8 resistors for 55c retail from Jaycar or 100 for around the same price from AliExpress. So it's in the ballpark.

CR2032 batteries are a common item with an order of magnitude markup (20c each vs multiple dollars here).

You assume it's the same product.

If you order electronic components from China, you're never quite sure what you get. You might get the stuff that failed QA testing at the factory, or second-hand stuff that's soldered off some old PCB. Or sometimes you get something else entirely... I ordered some LM35 temperature sensors in the TO-92 package, and got some cheap transistors instead. Case was TO-92, markings where as expected for LM35, but a quick test betrayed what was really inside.

In this case I expected them to be fake, based on the price, I just wanted to see what I got. But even if the part is appropriately priced you can't be sure.

A bare atmega328p-pu is 10x the price. LEDs are about 15x. Resistors and caps are, as another poster said, huge, but you're possibly getting factory seconds from an already mediocre brand. I buy decoupling ceramics and 5% resistors from China for peanuts but stick to RS online for caps where I depend on characteristics or reliability.

China sells a lot of older stock via AliX, so newer chips are hard to find and might be near the same price, but older clonable classics like a 2n7k are dirt cheap and just as good.