A Greenbelt realty firm has won an antitrust suit against a major housing cooperative that tried to force its owners to sell their units through the co-op rather than through real estate agents.

Jugde Vincent J. Femia of the Circuit Court for Prince George's County last week ordered the co-op, Greenbelt Homes, Inc., to stop requiring its members to pay a $550 "administrative fee" for the right to sell their units. It also ordered the co-op to pay plaintiff Eric Wade Barber, a member-owner of Ghi as well as a sales agent for Nyman Realty, Inc., located near the co-op, triple damages amounting to $1,005.

Greenbelt Homes is the largest co-op in the county. Its one and three bedroom rowhouses were built on a 250 acre tract during the 1930s and early 1940s. There are no vacancies in the units, which sell for an average of more than $20,000.

The suit was filed in June 1978 after the co-op raised its administrative fee from $150 to $550.

The court found that the co-op had "abused its market power to disrupt fair and honest intrastate competition" and enjoined it from continuing to charge owners fees that combine the costs of its real estate sales service with the its administrative and inspection costs.