Well the problem is if you look at Kelly Blue Book, you'll see that the car is worth more than $5000 even with $80000 miles on it. I looked up a 2007 Chevy Cobalt Ls Sedan which is the base model with 80,000 miles. I looked up the retail value in good condition and it books $5700. The best thing you can do is keep your eyes open on craigslist and eBay. The deal your looking for is one that doesn't come around too often. Here is a link to the one I looked up :
http://www.kbb.com/KBB/UsedCars/PricingReport.aspx?YearId=2007&Mileage=80000&VehicleClass=UsedCar&ManufacturerId=9&ModelId=42&PriceType=Private+Party&VehicleId=83524&SelectionHistory=83524|30905|80013|0|0|&Condition=Good&QuizConditions=