However, I find it hard to believe that because of that QA, a 0.04$ zipper now costs 10$. And the fact that the 60cm zipper costs 10$ and the 70cm one costs 12$ tells me that the local stores (or whatever intermediate responsible for the price) are simply abusing.

In other words, it feels like there is a responsibility on the side of the local stores: I want a zipper, I can't afford 10$ for one, so either I don't get a zipper at all, or I get it on Temu. In any case, the local merchant is screwed. Now if I get a zipper on Temu, suddenly I see all the cheap crap I can order from there, and I presume my local merchant abuse on their price in exactly the same way (which may or may not be right). So the local merchant "pushed me" towards Temu with their abusive prices.

Again: stock keeping costs money. That is what you primarily pay for. Real estate isn't cheap, especially not in urban areas, so a lot of products just get dropped outright and the products that move very slowly but can't be dropped (what would a sewing store be if it didn't carry a few zippers?) get marked up to pay for the space they consume in storage.

You'll have to do better if you want to justify that those 10cm are worth 2$.

I just chose a specific example (the zipper) where I think the local merchant clearly abuses, to say that it goes both ways. I'm fine paying more if it helps the local merchant, but I am not fine getting screwed by them. I won't buy a bottle of water for 200$, even if it's sold at a local store.