From the Man who sold you the Golden Gate Bridge

Now you can buy an acre of the Moon, Mars or Venus for someone you love from these people. The justification as to the provenance of title is fun:

------
So how on earth can it be possible to buy a chunk of green cheese up in the sky? Or a little piece of Venus or even a Martian acre? A UK company has bought the rights from the Lunar Embassy to sell plots of Moon. And just who are the Lunar Embassy we hear you cry? Well, you would think wouldn’t you, that after spending billions of dollars to get there and sticking in the American flag, the USA would hold all rights to the Moon. But no. Thanks to The Outer Space Treaty of 1967 and the Moon Treaty of 1984, both they and the Soviets, or in fact any other Government, was prevented for asserting their rights to extra terrestrial satellites. What they forgot to prevent however, was any other company (or indeed, group of companies), or individual persons from doing the very same thing and though several years passed during which time they had the right to amend the Treaty, they never did.

Thus it was possible for an American (Dennis Hope) to have the foresight to make claim to the surface of our whole solar system, in 1980, for himself and then sell large areas of it off to other companies for them to be in a position to sell the Moon, Venus and Mars chunk by chunk. Believe it or not but all this was perfectly legal and you will actual own the rights to the acre of moon you buy. This could of course prove very valuable to you or your descendants once we run out of land down here on Earth. Preposterous and cheeky it may be, but no-one can stop you. And that in itself is a huge joke. As a result this idea is taking off like a rocket and to date over 100,000 plots have been sold. The land is carefully mapped on a database and no plot is ever sold twice. In fact the agents have done everything they can to ensure that the process is fair and legal.
------

In fact the Moon Treaty - which applies to all celestial bodies - bans any ownership of any extraterrestrial property by any organization or person, unless that organization is international and governmental. But as the treaty has only been ratified by a handful of states, and those not spacefaring, it achieves nothing (at least until it is ratified by the superpowers, which is not likely).

I wonder how Dennis Hope (who presumably has never been to the moon, and certainly not to Venus or Mars) has 'asserted his claim' to these planets? First he would have to establish his own micronation. Then he would have to ensure a permanent presence of citizens from his micronation. A post office is good as are your community giving birth in the place in question (Argentina has been doing this in Antarctica, by shipping in a heavily pregnant woman for just this reason).

You can see the whole advert here.