Some are as you say - you order something and never receive it. Straight up
Some are fast fashion companies which get spun up quickly with slick ads and websites, which are somehow "going out of business" even though they were registered less than 2 months ago.
Some are kickstarters for products which obviously are just clones of other products which already are ubiquitous.
A lot are drop shippers who use slick ads to sell you something they aren't even in possession of, at 2-3x the price which you could get it for if you just took the time to look elsewhere.
Oldest trick in the book.
They said they're selling (furniture, electronics, clothing, art) it all for 80-90% off retail. You know, just so they get it off the truck and get back home. I remember working in a commercial area and the owner of the business would come and tell the guys to GTFO their property so they'd pull out and then pull into the next driveway over and rinse and repeat until they were out of the area.
It was a total scam though. My boss bought some couch and said it started falling apart after a few months. I'm not sure these guys had stolen all the stuff, or if the truck got hijacked our what, but during the Summer it was a regular thing that happened a few times every month.
I'd be interested to find out if anybody else remembers these "truck load" scams in the late 90's early aughts.
* I see an ad that says "do you want to buy this item for this price?"
* If I like the price, I can buy the item. I got what was promised.
The fact that maybe I could have bought the same item elsewhere for less doesn't make it a scam. Lots of things are offered at different prices in different places at the same time. That's not a scam.