Are they obligated to review every product in a category? The few review sites that I follow don’t seem to cover anywhere near the full breadth of SKUs, but the N most popular, which would become self-fulfilling if this was the primary mode of discovery.

The people who care would go to more specialized third party sites that covers more stuff. It already happens today, and if we shifted resources from marketing to such efforts they would become more common and popular.

We might be thinking about a very different scale of problem.

Search “usb 16gb flash drive” on AliExpress. Who’s going to meaningfully review all those results?

If we (effectively) don’t allow new entrants, that seems like a massive giveaway to Sandisk, Samsung, Kingston, and the other existing players.

I don't see how all those results are better of with Sandisk, Samsung etc constantly bombarding everyone with their marketing material. It isn't like you see ads for the small players here.

What are you seeing when you see those search results if not ads?

Sure it’s not a 30-second TV spot or 15 second pre-roll video, but there is money changing hands from the seller to AliExpress in exchange for that relationship (same as for Amazon, EBay, Etsy, Tindie, Newegg, etc...)

When you go into a grocery store and see Tide on the entrance end cap this week where Coke was last week: that’s a paid ad.

No they are not. Product listings are not marketing, now you are reaching at straws. Technically they could be considered marketing, yes, but in practice that isn't what people talk about when they discuss marketing. Bringing that up in a discussion about ethics of marketing is bad faith.

If I pay to place my product in a catalog (paper or online) where it will get exposure to shoppers who will hopefully buy it, what else would you call that expense if not marketing?

What you’re calling a bad faith argument is a straightforward use of standard business terminology as far as I can see. I’m not even bending the definition a little bit.

You can always go to the Sandisk website and browse their full catalog?

Of course I can go to the website of the biggest brand-name company I can think of and look through their catalog.

How does that help me learn about a new entrant to the flash drive market (or help them get me to see their catalog), though?