It's the type of sale that you come to the premises to do. For example if you want to buy a ticket to a certain function, there are two ways: you either order by phone to book your ticket (or the internet etc), or buy at the door.

This is called door sale. So you can translate it as either:
بيع/شراء عند الباب
أو
بيع / شراء في الموقع