If seller on Aliexpress (or eBay, but from China) is not representing official factory or big brand, as around 90% of them are - most of the good ones are not just dumb resellers.

What I mean here? I mean that seller can make lot of work selecting 1 factory among 30 who make extremely similar looking product. Many large sellers have dedicated inspector on factory who is inspecting each batch, as factories like to make some savings by cutting manufacturing costs or can try to dump some defective batch originating from other big order:-) Some of sellers make quite small or even deep customization of product for their needs - make sure to check this, sometimes it makes perfect thing from "so-so" general factory product.

Life of large seller is usually simpler, as they can push factories more and get better offers (still not like Walmart or Apple:-) ). But this also means that frequently they are pushing too much to get best price as best price on aliexpress gets you much more orders. Some medium sellers will offer worse price item, but with better quality.

Same goes for shipping and bad tricks. If you see seller who try to get best offer by using unreliable and cheap shipping - stay away, it'll be usually all same in other departments. Similarly for cheap bad tricks, like putting another products in the options to get low displayed price (btw, all marketplaces do this intentionally and not close this simple hole!)

Complex, specialized products are especially important. As online they can look very similar. But in reality factory and bad seller can cut the costs literally ruining the product, as you can't check internals or materials, or thickness.

Remember that looking at the "same"simple thing like small screwdriver you can actually see product made by 40 different factories and each of this factories can also offer at least one cheapened version with much worse material used for bits.

All online marketplaces are still in their very early, baby age. Most of them are still growing quite fast. But it will end. And as it'll happen, real long term reputation, some kind of quality inspection and independent reviews (financed not be manufacturer or seller!) will become standard part of each of them.