When the sites ask for the condition of the vehicle you're researching, categories also differ. Edmunds has five categories, while Kelley and CarsDirect have four. And the definitions of what constitutes "poor,'' "fair,'' "average,'' "clean'' or "excellent'' vary from site to site.

So what's a car shopper to do?
Information is still power, but in this case it also should be tempered with another bromide -- if it seems too good to be true, then it probably isn't true.

Disappointment is likely if a shopper holds out for only the highest trade value and only the lowest retail price. Also, no two used cars are alike.

Another thing to keep in mind is that on newer vehicles the various sites were closer together in establishing values. That could be because newer vehicles coming off lease contracts have more stable values that can be more easily determined.

Although Rosten defends Edmunds' prices as the most accurate out there -- as does Kelley and CarsDirect -- he says, "You're never going to be 100 percent accurate.''

Consumer Union's Gentile agrees: "There are a lot of things that go into a used car's value, including regional differences, supply and demand and what's happening in the new car market.

"At the end of the day, a used car is only worth what you're able to command for it.''

So check as many sources as possible to define a range, and work within that.

Three examples of how car-pricing Web sites compare *

1989 Toyota Camry LE
Four-cylinder, automatic transmission, AM-FM cassette, moonroof, anti-theft system, keyless entry, anti-lock brakes, 80,000 miles, average/fair condition.

2002 Ford Explorer XLT
Four-door two-wheel-drive, V6, automatic transmission, AM-FM cassette, leather, 50,000 miles, average/fair condition.

2003 BMW 325i
Six-cylinder, automatic transmission, Harmon-Kardon AM-FM/CD, leather interior, power driver seat with memory, moonroof, navigation system, 30,000 miles, excellent/outstanding condition.

* These were the prices posted on the Web sites on Dec. 2, 2005
** The CarsDirect.com prices came from the site's used-car prices link and not the trade-in appraisal link, which doesn't give private sale or retail pricing.

Terry Jackson is the author of six automotive books and has been writing about cars for 25 years. He is the former editor in chief of AMI Auto World Magazine.