What you say would be true if the West Bank were "occupied territory" instead of "disputed territory" under the law. Check it out. You might also find out what "terra nullius" means, while you're at it. Aside from that, there is no international law that prohibits private individuals from purchasing land wherever they find land for sale. A Russian national can purchase property in Paris or Denver (to use your example) and becomes the owner of that property, regardless of the nation it's located in. That doesn't mean that the property is automatically Russian territory.
Israeli teen lightly wounded from stone throwing in West Bank settlement of Kiryat Arba (Haaretz)