If you can't afford to buy a whole thing, buying a bit of it might be just as good. A person can't sensibly take off with an entire ship if you own the rudder - that'd be stealing your property. This is a good legal loophole you can use to restrict people from absconding with something you don't want absconded with. And besides preventing movement of the property, you could also make using the property difficult or impossible by choosing a vital component.
GMs will want to try it around the other way. Sell the heroes a castle, but the mad wizard who was the original owner wants to keep ownership of the mysterious tower, which must stay locked at all times. And woe betide anyone curious enough to try to break in...
Luke: I'm going outside to think.
Han: Yeah, I often need to do that too.
Chewbacca: Ah, that may explain your behaviour when confined to a spaceship.
Han: Hey... let go of my leg, will ya?
Wicket: No, it's mine.
Han: Whaddya mean it's yours?
Wicket: I bought it before you were released.
Han: But we were released.
Wicket: Without my consent. I still assert my ownership.
Chewbacca: Why just his leg?
Wicket: What do you mean just a leg? It'll feed my family for a week.
R2-D2: Roast leg of Han. Mmm-mmm.