As DkBG notes, when you bought the car originally, you likely checked the mileage. The basic issue with a car is that the value at any time is a reflection of a few factors: 1.) Supply and Demand, 2.) Condition (and mileage is probably the greatest consideration, everything else being the same), and 3.) "Market Depreciation". Supply and Demand is fairly obvious. The recent Zero-Zero ($0 down and 0%) programs by GM (and through December 31, 2002, you got 90 days with no payments...) has killed the used market. Given the same condition (e.g. Excellent), a car with 10,000 miles will be worth more than one with 50,000. Lastly, Market Depreciation is a function of the continuous improvement in features that occur from year-to-year. A few exampleS: LS6 engine; Active Handling; Magnetic Selective Ride Control, and, and, and....
While you will get the best price from a private party sale, you will incur some costs that may minimize the difference between the price that you would get in a trade-in: you will have to advertise the car; you may have to go through a series of iterations with potential buyers (filtering out the not really serious buyers from the serious buyers; the highest offer that you receive may not be what you are looking for; etc.); and, if a potential buyer wrecks the car while on a test drive, you have another problem entirely....
If it were me, since you have had a very good ownership experience, I would continue to enjoy the car until you are ready to move into another one. Keep doing the maintenance; the single best action that you can do to lengthen the life of the car is the preventive maintenance.