If you ask someone, "Are you here to buy stamps?", that suggests that you work at the Post Office. However, you would not then go on to ask them if the stamps cost different amounts. I don't know why either of you would ask "Why?" as any part of this dialogue.
My suggested dialogue is:
Post Office employee: Would you like to buy stamps?
Employee: Which stamps would you like?
Me: I don't know. How much do they cost?
Employee: £1, £2 or £3.
Me: OK. I would like one £1 stamp and one £3 stamp please.
Student or Learner