Temná ulička je jediný "trh" ive Staré Wyzimě. It's pretty completely during the day, but at night a few fences roam the area and will grudgingly do business with anyone, even witchers. They'll buy and sell almost anything. You can find, or more importantly, convert to cash:
volba strany affects the fences. When you first arrive in Stará Wyzima, there will be at least one neutral (blue) "Překupník" in this area. Upon returning to Old Vizima from the Hřbitov v bažinách, the fences will not be there unless you're on the neutral path. On the Scoia'tael path there will be a "Zásobovatel Scoia'tael" near the gate to the Dike instead. On the Order path, there will be an "Zásobovatel Řádu" near the gate to Hřbitov v bažinách instead.