“There are all kinds of ways the people try to manipulate the system to their own financial gain,” said Jack McCabe, an independent real estate analyst with McCabe Research and Consulting. “And you are going to see it here because Florida is the capital of real estate fraud.”

Mr. Guerette, who now faces up to 15 years in prison, insists that his business is legitimate and moral. He said he got started last year, driving around working-class neighborhoods in Palm Beach and Broward Counties, looking for a particular kind of home: not just those with overgrown lawns and broken windows, but houses with a large orange sticker from the county reading “public nuisance.”

The stickers signaled owners out of touch: the county or city was unable to reach them.

Mr. Guerette filed court claims on around 100 of these properties, which appear to be in the process of foreclosure. Then he chose 20 that could be most easily renovated and sent letters to the owners and their banks  presumably overwhelmed  to make them aware of his plans.

Florida does not require notification. One state lawmaker tried and failed to close that loophole last year with a bill that never passed. But it hardly mattered. Nineteen of the owners and their banks did not respond, Mr. Guerette said.

So he set about fixing up the unclaimed properties. In some cases, he just mowed the lawn and replaced stolen air conditioners or broken windows; in other cases, like with Mr. Ferguson, he let tenants make improvements in lieu of rent.

At his peak last year, he said he managed 17 homes with renters, some of whom he found on Craigslist, others through a Christian ministry in Margate, Fla.

Copies of leases show Mr. Guerette included an addendum noting that he was not the legal owner. Tenants like Mr. Ferguson and his family, who had been homeless before moving in last year and paying $289 a month, see Mr. Guerette as a savior.