Looking for 300 parking spaces conveniently located midway between downtown Silver Spring and the District of Columbia line? The Montgomery County parking agency may have a bargain for you.

The county Transportation Department's parking division has declared 300 of the 612 spaces in its No. 4 garage temporarily surplus, and has invited interested parties--possibly new- or used-car dealers, car-rental firms, or companies needing employe parking--to submit proposals.

The 1968-vintage garage is at 3110 Fenton St. near Silver Spring Avenue, east of Georgia Avenue, in a section of Silver Spring not far from the B&O Railroad station. The area has become something of an economic backwater in recent years. Parking there still costs just 10 cents an hour, compared with 15 to 25 cents just a few blocks away in downtown Silver Spring.

"Not as much development has taken place down there as anticipated," is the way Joseph L. Tracy Jr., the parking division's chief, put the garage's problem of under-utilization.

The county has set no minimum price for the 300 spaces. "Whatever proposal comes in we're going to evaluate," Tracy said.

Whoever rents the spaces, however, may not park "wrecked, junked or otherwise inoperable" vehicles and may not use them for selling or repairing the vehicles, according to the parking agency's official announcement.