Trinity Smith lost her home in Detroit because of unfairly high property taxes—and in the process, she handed over thousands of dollars to a multibillion-dollar money machine that benefits real estate speculators, big banks and the county government that foreclosed on her.

Smith’s home was one of more than 100,000 that officials have auctioned off over the past decade in Wayne County, Michigan, which has transformed delinquent property taxes into investment opportunities. Since 2005, county officials have used the debts to back roughly $3.5 billion in bond sales—securities that pay high yields to investors and are funded by penalties, fines and foreclosure sales like Trinity Smith’s.