All of those have value even if you wouldn't be allowed to sell them. Stocks pay dividends, people use gold as jewellery, and you can live in real estate. Speculating on assets with real world value is different from speculating on assets nobody would care about unless they could sell it.
The whole point of these scams is to convince people to buy assets that nobody wants, but you convince them that some other guy really wants it a lot and will pay more for it in the future. And of course if he could convince some new guy of the same thing he can even sell it at a profit, but since there ultimately is nobody who wants this asset the chain has to break sooner or later.
I described attributes of a Ponzi scheme. These attributes may apply to other investments. I made no attempt to define anything. You infer where inference is not applicable.