Starting the "back in my day rant": In the past, you would actually negotiate with your manufacturers about how many items you would purchase over the year, and that would determine the discount you got on their SRP. You could then mark it up, but for the most part you didn't, and your profit was your discount.
Now days, though, everyone is just plugging directly into AliExpress/Temu/Wish, and marking it up. They are just a lazy low/no-value middle-person between me and cheap knockoffs from China. The "clever" ones are worse, because you might not even realize you are buying a knockoff. The packaging, images, titles, and descriptions will be 1:1 with the real product.
We are also better equipped to guarantee supply chain integrity and have much better customer service. Getting a warranty on a part can be difficult, but we have dedicated people who work with vendors every day. All the customer has to do is stick a shipping label on box.
Or for that matter, car-dealerships operate on the same model.
1. Platforms could detect they're sellng the same piece of good and automatically sort by price
2. There's no material difference between vendors other than pricing and offering, e.g. in customer service
3. vicious anti-competitive behaviour like those illustrated here are heavily regulated
When all these are true the vendors that survive would be ones that offer the best price among those who offer goods of the same level of crappiness.
No matter where you go and buy a fungible consumer product (SKU), the retailer was regulated by law to give you the best price available anywhere in the world at that exact microsecond.