The used car that Phyllis Weifenbach bought yesterday had been worked on by mechanics too young to vote, sold to her by a salesman too young to drive, and displayed on what may be the only nonprofit used car lot in the country.

"I thought the salesman was just darling," Weifenbach said, referring to Mike Smith, 15, the part-time saleswoman who is also a Wheaton High School student. "And the sales prices was $15 below the Blue Book price," she said.

Weifenbach paid $2,120 for a four-cylinder 1976 Pontiac Astre at the Montgomery County Public Schools Automotive Trades Mini-Dealership, which held its grand opening, complete with coffee and cookies, yesterday.

The minidealership, which started with seven late-model cars to sell, is being run as a part of the vocational training program of the county school system. Currently, 38 junior and senior high school students are involved part-time in learning the repair, sales, and accounting procedures needed to run a competitive, full-fledged used car lot, officials said.

The profits realized from the sale of the cars, which are priced competitively with vehicles on other area lots, will be plowed back into the dealership, which has been set up as a nonprofit foundation, according to the president of the foundation.

"This is a good investment for us, for the community," said Harry Martens, the foundation president who is the owner of Martens Chevrolet in Silver Spring. "There is a crying need in the industry for good technicians."

Montgomery County Council President Elizabeth Scull, who was present at yesterday's opening, said. "The dealership addresses the need for vocational training and alternative forms of education. Just sitting in a classroom is not meaningful for some students. This gives them confidence in themselves and the feeling of being useful."

Patterned after the successful nonprofit corporation set up last year by the school system to build and sell $100,000 homes in the county, the dealership buys used cars in the open market. They are then brought to the foundation's garage at 590 Stonestreet Ave., Rockville, for refurbishing.

"All the cars get a tune-up, oil change, oil filter change, transmission fluid and filter change, whatever is needed," said Michael C. Wilson, coordinator for the automotive and construction trades projects.

Irving Fisk, the Montgomery County schools instructor who works with the 23 students involved in automotive repair, said that "two shifts of students go over the car, one shift in the morning, one in the afternoon. They check each other's work, and then I check it. We don't want someone coming back in here after 4,000 miles needing a brake job," he said.

Fisk added that all cars are also inspected by the Maryland Department of Motor Vehicles, as are all used cars offered for sale.

Wislon said that eventually vocational students will become involved with automobile air conditioning and electrical systems, and radio work.

"My auto mechanics class was getting boring," said Dale Moyer, 16, a student at Northwood High School. "All we were doing was brake jobs and tune-up. This will help me when I get out of school."

Michael Smith, 15, the salesman who sold Weifenbach her car, said, "I told her this was a great car, in nice condition, five speeds. She said she wanted it for her daughter, and I said that's great. There's possibility I'll go into this after school, I don't know.'

Information about the availability of cars can be obtained by calling 279-3434. The lot next will be open to the public this Saturday.