Often the notice comes as a flier in the mail, but you might get a friendly phone call from the salesman who sold you a new car a few years ago.

The dealership is paying top dollar for trade-ins, you're told, and now is the perfect time for you to sell your vehicle. In sum, your new-car dealer really wants your used car.

There are about 260 million cars on America's roads. Why is your 3-, 4- or 5-year-old machine so special? It's because, for the car business, these are unusual times.

The changing car market

According to the U.S. Bureau of Transportation Statistics, 10.6 million new vehicles were sold or leased in the United States in 2009 (the latest year for which the bureau has reports), while 35.5 million used vehicles swapped hands.

That 10.6 million new-car market was a big down year for the industry -- down from 13.5 million units in 2008, 16.5 million in 2007 and more than 17 million in both 2006 and 2005. Used-car sales also dropped between 2005 and 2009, but only about half as much on a percentage basis.

Car sales have recovered somewhat since 2009 (despite the effects of Japan's tsunami, which has restricted availability of some new Hondas and Toyotas). Edmunds.com projects that 2011 new-car sales are likely to come in around 12.9 million units, and, according to the Manheim Auto Auctions, just under 37 million used cars swapped hands last year.

The used-car market has always dwarfed the new-car market in unit sales, and many, if not most, dealers make more money on their used-car sales than they do on new cars. But when new-car sales are down, dealers have fewer used-car trade-ins to sell.

So with 2009 new-vehicle sales representing about 60% of what they were in 2005, up to 40% fewer used cars were being traded in.

Those declines have meant there are a lot fewer of the prime, low-mileage, late-model used cars around to sell in dealer lots. In particular, with gasoline prices higher this year, there aren't a lot of late-model small used cars such as the Honda CivicToyota Corolla and Ford Focus around, and demand for them is up.

"Strong consumer demand for small used cars is driving prices up," Jonathan Banks, a senior analyst with the National Automobile Dealers Association Used Car Guide, said in a news release. "The NADA Guide increase in trade-in values for June should come as no surprise because it reflects a shortage of both new and used cars entering the market."

In May, NADA projected trade-in values for small cars to rise 18% during the first half of 2011, compared with the first half of 2010. The differences in value can be startling. Even in one month, values for used small cars have skyrocketed.

For instance, the NADA Used Car Guide had the trade-in value of a 2009 Kia Rio at $6,400 in May and by June had raised that to $7,500. That's a 17% rise in just one month. You're not getting that kind of return on your savings account. Even the value of something as old as the 2005 Subaru Impreza RS is rising, up from $6,650 in May to $7,575 in June.

Values are up so sharply for some cars that it's tempting to trade in a car that's only about a year old. For instance, a new2010 Honda Civic DX coupe carried a $15,455 sticker price (plus taxes, license and delivery). Today that same car has a NADA trade-in value of $14,075. That's a drop of only $1,380 over a year.

"A lot of lessees and buyers will find they're on the right side of their leases or loans," Banks told MSN Money. "If they trade in their cars, they'll find they have some money for a down payment."

It's all about supply and demand. Dealers want your car because they have buyers for solid late-model used cars lined up, and there simply aren't enough cars for them out there. And they may as well try to get your car now, before it's worth even more next month. Plus, they wouldn't mind selling you, the supercreditworthy person that you are, a new one to replace it.

You are supercreditworthy, aren't you?