Last I knew, making an offer to sell something you don't own — and don't expect to own — was considered fraud. Even when it's the Moon.
This particular "entrepreneur" er, fraudster, one Dennis Hope, has been claiming ownership of not only the Moon, but seven of the [eight or nine, depending on your opinion] planets in the Solar System, plus their moons, for over 20 years. That just makes him a harmless loon. However, the 400 million acres of Moon real-estate he's "sold", at $20 an acre, are another matter — especially since he's declared he allocates properties by closing his eyes and pointing at a map of the Moon. So, let's see, he's selling unsurveyed property that he doesn't own, without plats or markers .... and so far he's made $9 million doing it. (Unsurprisingly, no government on Earth recognizes his lunar property deeds as legitimate or legally binding.)
I expect most of his buyers are "buying" in fun, of course. But unless he has some kind of "for entertainment only" disclaimer on the 1,500 lunar property deeds he claims to sell every day, his ass could be in a pretty serious sling if someone ever decides to go after a chunk of that $9 million by filing a class-action suit against him for fraud. Last I knew, "It was just a joke, everyone knows I wasn't serious" isn't a valid defense in court.