You can also have a look at the Duchy website:
Duchy of Cornwall - Home Page - The Official Website for the Duchy of Cornwall
Usually, for property owned by something like the Duchy of Cornwall or the Crown Estates, people can buy the property leasehold rather than freehold, which means they're effectively buying a long lease on the property. They can sell the property, but they're selling the long lease, not selling the property outright. I'm not sure how the properties in Poundbury are sold; this article mentions something about both leasehold and freehold property:
CABE - Case Studies