I think I found the seller you're talking about. In one link, it looks like the betta is actually in a bit of water. There is what looks like plastic under him with some water. Probably not enough for him to move, though.
In another ad, I see the betta actually in a plastic bag. In fact, in almost all of the ads, they're in a small plastic bag with little water.
I'm not trying to defend the seller in any way, and I don't even know if it's the right person. I do see where you're coming from, and the fish do look stressed.