TIANJIN, China—Thirty floors above the showroom of a Chinese developer, a 29-year-old woman stood on a small rooftop ledge about 8 feet off the rooftop itself, threatening to jump and declaring that her recent home purchase had ruined her life.

Ms. Hou, who asked to be identified by only her surname, was one in a group of angry home buyers who had gathered at a real estate sales office in Tianjin, a port city about an hour from Beijing, on Saturday, demanding their money back for half-constructed apartments that had now dropped...