They can't sell you bits of the moon or Mars, because the moon and Mars do not belong to anyone. They can, however, sell you a certificate which tells you that you own a bit. Whether you want to buy such a certificate is up to you. Whether it's a joke or a con depends on how much you are willing to pay for such a thing.
cpl Fraser, sometimes, often, they're posting simultaneously, especially with long answers, else, i agree, overkill, but confirmation is good if not too wordy.. i sometimes put 'agree with so n so' :-)
It wasn't *exactly* the same. I was making the point that some people would willingly pay the money for a certificate, just for the fun of having one - if they want one at that price.