One such sale, for $25,500, was for a house in Elyria, Ohio.

In April 2011, Harbour sold a contract for deed on the house for $36,300, according to public filings. Under the terms of that deal, the 24-year-old woman who now lives in the house is required to pay 10 percent interest or a monthly base payment of $314 — a sum that does not include property taxes, insurance or any outlays for repairs or renovations.

Under a contract for deed, a buyer gets the legal title to the home either at the end of the contract, which can run anywhere from 20 to 40 years, or if the buyer is able to pay off the balance owed all in one go.

After the financial crisis, Harbour emerged as one of the larger national players in the contract for deed market. The firm bought more than 6,700 single-family homes, most of them from Fannie Mae.

But in recent months, Harbour has sold more than 600 homes to investment firms like Shelter Growth and individual investors, according to public filings.

A review of some of the homes sold to SG Capital found that the contracts used by Harbour have drawn criticism from some housing lawyers because the documents do not provide buyers with a specified time period to remedy a default, give Harbour the right to immediately convert the agreement to a month-to-month tenancy upon a default, and include an arbitration clause for settling some disputes.

Through his lawyer, Jacqueline Mallett, the founder of Harbour, Charles A. Vose III, declined to comment for this article.

It is unclear whether Mr. Sparks’s firm plans to change the contracts or abide by the terms. Mr. Sparks declined to comment.