“There’s nowhere else to go,” said Rar, who emigrated from West Africa about five years ago. “There’s no way to find a job. You gotta eat…. It’s better than you go rob people and go selling drugs.”Rar’s blanket of DVDs took up several feet of the sidewalk, which was already narrowed by the construction barriers separating pedestrians from the water main work on Fulton St. Whenever someone stopped to browse Rar’s DVDs, pedestrian traffic in either direction slowed to a crawl. A chief complaint among residents is that vendors on streets with construction make it impossible to walk.Beside Rar was Ous Mousa, 52, a U.S. citizen who emigrated from Mauritania 22 years ago. Mousa was selling $30 plastic-wrapped boxes of perfume that he promised were the real thing. Selling perfume helps him pay for his apartment in Brooklyn where he lives with his seven children, though he only occasionally makes more than $100 a day. His business has dropped by 75 percent since 9/11, he said. Mousa has a general vending license and said the police rarely bother him on Fulton St., since he is allowed to be there. But according to police, general vending is forbidden on Fulton St….