Believing she couldn’t afford Manhattan, Ayana Green was house hunting in Queens when she saw a StreetEasy listing for an East Harlem co-op. At $450,000, it was one-third of Manhattan’s median home price. Better yet, it was a two-bedroom with maintenance charges of only $585 a month. She saw the apartment and made an offer. One thing seemed odd, though. The seller’s broker urged her to use Midtown-based mortgage provider GuardHill Financial. “I was told during the showing that they ha...