On the teeming streets of Lagos, the Nigerian mega-city of 15 million people, the once omnipresent money-changers are going underground. They’ve become the latest target of authorities desperate to bolster the naira and crush a black market for foreign currency that’s boomed since the crash in oil prices strangled the inflow of dollars. This month, the central bank capped prices that non-bank dealers can charge their customers for dollars, effectively pegging the black-market rate, with intelligence agents threatening to jail anyone who doesn’t comply.  That’s creating a parallel market within the black market, according to analysts at Lagos-based CSL Stockbrokers Ltd. and Afrinvest West Africa Ltd. One trader in the Lagos suburb of Surulere, who asked not to be identified as he feared arrest, said he would continue trading at the old rate with trusted customers while refusing to sell dollars to others. Anyone he doesn’t know may be a government spy, he said.   ‘New Level’ “The black market will go further underground,” said Omotola Abimbola, an analyst at Afrinvest. “The fact they went as low as getting security forces on the streets shows a new level of desperation.” Nigeria’s interbank market sets the official naira exchange rate and is meant to serve businesses, but the foreign-currency shortage has forced many to the licensed bureaux de change and the unofficial, or black, market of informal street traders, which sell dollars at a higher rate. The central bank has made several attempts to defend the currency after it plunged to a record in 2014. Governor Godwin Emefiele tightened capital controls and restrictedbanks’ ability to trade foreign-exchange, then tried a currency peg that deterred foreign investment and worsened the shortage of dollars companies need to pay for imports of raw materials and equipment. As the economy shrank and inflation soared, Emefiele relented, resurrecting the inter-bank currency market in June. The naira slumped 38 percent within two months, prompting central bank intervention that has held it around 315 per dollar since then. The currency was little changed at 315.69 per dollar by 8:56 a.m. in Lagos.