Proposed is a method for carrying out a transaction between a mobile telephone (6) and a sales apparatus (45), the telephone (6) comprising a first memory (40) for storing an identifier (I), the identifier (I) being able to identify the mobile telephone (6) uniquely, the method comprising the following steps: a) reception of the identifier by radiofrequency communication via the sales apparatus (45), b) identification of the mobile telephone (6) by the sales apparatus (45) from the identifier received during the first sending step, and c) generation by the sales apparatus (45) of a proof of purchase comprising a signature computed by the sales apparatus (45) and able to authenticate the proof of purchase, the proof of purchase (PA) being a transportation title.