First off, use the True Market Value tool to "appraise" your particular car, putting in all the options, mileage, trim level, location, color, ect.
TMV will give you a trade-in value, a private party value and a dealer retail value.
For some back-up numbers, go to www.autotrader.com, and using their advanced search feature, search nationwide for Libertys with similar mileage and equipment (autotrader search allows you to customize your search).
Once you get a list of Libertys like yours for sale, scroll to the bottom of the page, and you will see a summary section that shows the number of cars that came up, the highest price, lowest price and average price.
Now remember, this is average ASKING price, so figure 5-10% lower for "real money"
in theory, this average price adjusted for real money, should come pretty close to TMV private party numbers.
KEEP IN MIND that when you use any of these tools, you MUST be sure you are researching your trim level and your mileage. You cannot compare a higher trim level or a car with 50,000 fewer miles.