The present invention refers to self-service sales counters used in commercial establishments that allow the customers themselves to check out their purchases and make the corresponding payment without requiring the intervention of an attendant, said sales counters comprising a first (20) and a second (30) self-checkout stations, placed in parallel to one another along a full-length partition (61), said sales counters comprising a first commercial automation module (63) associated to the first self-checkout station (20), a second commercial automation module (64) associated to the second self-checkout station (30) and a banking automation module (65) shared by both stations, wherein the control of each of said modules is performed by a specific independent program. The method for conducting transactions comprises the identification of each item of merchandise (41) by means of scanning of its respective identification code as well as weighing the item by means of a bioptical scanning scale (40), checking the authenticity thereof (45) by comparing this weight with the weight of the identified item as stored in a memory means, and checking the weight of the items of merchandise placed on a packaging platform or counter against the sum total of the weights of the already identified items (47). A hand-held barcode scanner (56) is used to identify bulky items, in which case no weight checking is performed. A list of already processed items is shown in a display, and the user may eliminate any items from said list if so wished.