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Why do people buy US dollars on the black market?
A falling exchange rate is an indication of falling confidence in a currency.  Countries like Iran or Venezuela, with a managed exchange rate, set their exchange rates at a higher value than the market accepts. Such market expectations may be influenced by poor government management, interventions into markets (such as nationalising businesses) or general instability / scarcity. The governments act to manage that uncertainty by limiting the availability of foreign exchange and pegging the exchange rate.   Since there is an inadequate supply of trusted foreign currency people turn to informal exchanges in order to hedge their currency risk.  This creates a negative feedback loop.  People in government who have access to foreign exchange start to trade on informal markets, pocketing the difference in the official and unofficial rates.  The increasing gap between the two rates drives increasing informal market exchange and can result in speculative bubbles. Driving instability (or economic contradiction) is that the massive and increasing difference between the official and market exchange rates becomes a powerful form of rent for government officials. This drives further state-led rent-seeking behaviour and causes the economy to become even more unstable. If you're interested in a more formal academic study of how such parallel markets in currency arise, "Zimbabwe’s Black Market for Foreign Exchange" by Albert Makochekanwa at the University of Pretoria is a useful source.