"Sir, when do you want to buy the houses?"
Translation:Señor, ¿cuándo quiere usted comprar las casas?
I think it usted comes before comprar because it makes it clear that 'you' is operating on the second verb comprar and doing the buying and not just the wanting.
In a different situation you might want to say 'Señor, ¿cuándo quiere ellos comprar las casas? If you wanted 'them' to buy it.
It does show that ¨usted¨is to be used. Señor, will not be mixed up with the informal ´quieres´etc---its like us normal people walking up to the queen and addressing her as ´old girl´or ´mummy´.
¨ustedes¨is also formal but is plural so we would have started with ¨señores¨
You have to use the "formal you" for every person that is not a close friend of yours, a family member or a child. And if you address someone by "Mr.", chances are he's not in that group.
You have to get used to using "usted" for most people you don't know too close or you would be considered a rather rude person. It's not only for "the palace".