Torgsin is a typical Soviet contraction for "torgovlia s inostrantsami" -- trade with foreigners. This was the name for these stores in the 20s and 30s. In theory, anyone with hard currency and valuables could enter this store and purchase unobtainable goods such as food and clothing. There were, of course, security guards at the door who would not let people in if they looked as if they did not possess any valuables. The Torgsin store was different from the Beriozka of the late Soviet period in that the Beriozka would only admit foreigners. Often, Soviets waited near the stores and offered to buy some of the 'exotic' purchases made by the foreigners as they left the store. The specific store mentioned is on Smolenskaya Square, at the corner of Arbat and the Garden Ring (Arbat 54/2), and still functions as a large supermarket today:

Sed'moi Kontinent store in 1998