If you build in a protected area (which is what I assume is meant by green belt - there are many different categories, so in general) it can only be sold to someone with links to the area. Look up the policy if you need proof that it isn't bollocks. Of course, everyone has the same right to build on land (me you and mr t) that's granted planning permission and if it has no protections can be sold at a profit (if indeed the valuation is more than the building costs). You realise that plenty of people that aren't gypsies build without planning permission all the time - there was a notable story about the guy who built a massive extension to his house and hid it with a wall of hay. After seven years it's automatically granted planning, as long as no one has noticed.
and if you put a caravan on land for a set ammount of years same rule applies