East Germany and Russia sold these and other WW2 German weapons around the world. E Germany made ammo for them, and it is still commercially made in several countries. A good (registered) StG here will bring anywhere from $25-50,000, depending on condition and how bad the buyer wants it. These are the rifle Kalashnikov copied the AK-47 from, or maybe Hugo Schmeisser, the original designer, who was held in the USSR after the war, designed it, and the Russkis gave Kalashnikov the credit as propaganda.
I’ll never believe Kalashnikov designed the AK. Prior to that, his most complex construction was a device that counted the number of rounds a tank had fired.