You're basically assuming that the company can't be made to turn a profit, in which case, yes, it would be a scam.
But that's not the case. The company could potentially be made to make a profit, and you're basically selling that potential. It often works out, like in the case of Amazon. Sometimes it doesn't, like Yahoo buying Tumblr.
As long as what the prospective buyer is actually getting is clear and up-front, it can hardly be a scam. With your "You sell something you know to be worthless to someone too ignorant to understand that.", you're essentially assuming the company can't be made profitable, and that the seller knows that, but doesn't disclose it to the buyer, and that the buyer is somehow naive enough to not be able to tell.
It's generally unlikely that a company can't be made profitable, it would be unlikely for the seller to know that, and it would be unlikely for the buyer to be unable to find out before buying it, which altogether, makes it unlikely this would happen. Which is why it's big news when it does happen, like with Theranos (Which was eventually found out)