In a self-service store, the customer goes from shelf to shelf, selecting and putting items in a basket. He comes to a check-out counter to pay and take the goods away. A sale contract is formed in the course of he coming in the store and going out. When and where a contract is formed is an important question for the rights and obligations of the customer and the store. The technical note explores this question and extends it to the new development of the replacement of the check-out counters by self-check out machines.