Buying a used car can be a sweet deal if you look out for lemons

Mention ''used cars,'' and many people will conjure up a vision of a slick salesman in a flashy sport coat who will grab your money and sell you anything that rolls out of the lot.

An increasing number of people, however, seem willing to take the chance and are choosing a used car for their transportation needs. In 1980, Americans bought 8.7 milion new cars; but used-car sales hit a record 18.6 million.

For many of these people, the phenomenon of ''sticker shock'' that comes over them when they read the prices on window stickers of new cars is enough to send them to the used car lot. Despite the pitfalls, the lack of a year-long warranty , and the lack of the thrill that comes from driving home a chariot with that ''new-car smell,'' there are a number of advantages to buying a used car. Almost all of them have to do with money.

* The price of a used car, particularly one of the big ''gas guzzlers,'' can be more than 50 percent less than a new car. If you finance it, you'll notice the difference in your monthly payments.

* A used car is cheaper to insure.

* The biggest drop in the value of a new car through depreciation occurs in the first year. So when you sell the car again, you'll probably get a bigger share of your investment back.

* If you exchange cars every three or four years anyway, a good, well-maintained used car should last that long, if you keep it in good shape, too. So you may not need to spend the extra money on a new car.

Against these advantages is the major disadvantage of uncertainty: You're never totally sure of the condition of the used car you buy.

But if you have decided to buy a used car anyway, the best consumer protection agency you have is yourself. Last year, the Federal Trade Commission proposed a rule that would have required used car dealers to put a disclosure sticker on the cars. The sticker would tell people about any defects the dealer was aware of and what, if anything, had been done to repair them.

A congressional committee voted in December to kill the FTC rule. Both houses of Congress, however, must veto the ruling within 90 legislative days after it is published by the FTC. The agency will resubmit it when Congress reconvenes later this month; but based on widespread opposition to it in Congress last year , many observers do not think it will survive.

What, then, can you do to make sure your used car isn't a lemon?

First, be careful where you buy it. If you buy from a used car dealer, it is usually best to start with a new car dealer who sells the same brand of automobiles new. He may have even sold the car to its first owner and his mechanics are familiar with it. You're also more likely to get a stronger warranty.

In addition, dealers usually keep the best cars that are traded in, and sell the others on the wholesale market. In addition to making mechanical repairs, some dealers even send the cars to a body and paint shop for a ''new'' look. As long as body work wasn't done to cover up something - like a vinyl roof to disguise a retired taxicab or police car - it's OK.

Buying directly from the previous owner may or may not get you the best deal. He may have maintained the car poorly, and done no pre-sale repairs. And there is no warranty. On the other hand, the previous owner can tell you how the car was driven and maintained, and will probably charge less, since the middleman's profit has been eliminated.

Used car dealers who have little or no service facilities should be last on your list. They may have had their mechanic make some repairs before the sale, but most offer no after-sale service and a very limited warranty, if they offer a warranty at all. If you do shop at a used car dealer, ask how long he's been in business. The shady operators tend to move around often.

Some states have laws stating that cars cannot be sold ''as is,'' and that used cars can be returned if they fail to pass inspection or fall apart very quickly. Check your state laws to see what your rights and protections are. You may have to remind the dealer, who has ''forgotten'' some of the details.

When you do decide to put yourself in the used car marketplace, look for something else: a good mechanic who will either go with you to the dealer or will be available if you want to drive the car to the shop for a thorough check. If you hire a mechanic, you'll pay up to $50 or $60 for the service, so you'll want to narrow the list of cars for him to look at.

In the lot, you can check for things like rust (especially check the wheel wells, floorboards, and trunk), worn tires and upholstery, and loose foot pedals. You can also check the electrical equipment, including lights, horn, and radio. Also look for holes in the exhaust system, leaking transmission fluid, and windows that don't roll up and down. Oh, see if the engine starts, too.

You should insist on a test drive that includes slow and moderate speeds on the streets and faster speeds on the highway. Here you can find out how the car accelerates, listen for odd rattles, see if the steering is too hard or has too much ''play,'' feel how the car holds the road on corners and curves, and check the brakes. If it is a manual transmission, see if the clutch ''slips'' when you accelerate, and if the stick has too much ''play'' for your liking.

Finally, especially if you buy from an individual, ask to see complete records of ownership, to be sure the car isn't stolen.