Sándor Lau, once a filmmaker, found his calling on YouTube when he came across instructional videos about how to buy seemingly worthless home loans.

Mr. Lau, of Portland, Ore., started accumulating mortgages, some of them delinquent for many years, dating back to the housing bubble. He owns more than 100 now, he said, with millions of dollars in payments owed to him. His full-time job is trying to collect.

A decade after the financial crisis, there is a new breed of risk-takers in the U.S. housing market. During the boom before the bust, lenders made mortgage loans to countless buyers who couldn’t afford them. Lenders later wrote off many of the loans, but borrowers’ obligation to pay remained. Today, in an improved economy, a rag-tag group of individual investors, plus some Wall Street giants, is buying these old loans and trying to tease value out of them.

The trade goes like this: Buy mortgages available for very low prices because no one has made a payment on them for months or even years—nonperforming loans, they are called. Track down borrowers and let them know they have a new creditor. Tell them they need to resume payments, at least at a partial level, perhaps offering to modify terms. Threaten foreclosure if necessary. If all goes well, collect on the debt or resell the mortgage as a now “reperforming” loan.

The process marks a new chapter for hundreds of thousands of crisis-era borrowers who often had heard nothing about their unpaid loans for years and thought the debt had been disposed of. For investors, legal wrangling with such borrowers is common.