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