Usted and ustedes use the 3rd-person conjugations, i.e. the same forms as él/ella and ellos/ellas, respectively. It's done to create a "respectful distance" to the addressee.
Para is used here because the people receive the table. If you said "por cuatro", it would be read like you want to buy the table in exchange for four... Pesos, probably.
Rule of the thumb: if you wouldn't address someone on a first-name basis, use the usted form.
Is it more polite to say "Tiene usted . . . ?" than "Usted tiene . . . ?"?
Not a matter of politeness. It's just more common to invert the subject-verb order in questions where the subject is a personal pronoun. "¿Usted tiene...?" sounds a bit more like "And you, do you have...?", putting more emphasis on the person.
How can I get it wrong for stating "you all" but it's correct in another lesson?