Trade-in price is the price that the dealer give to customer if you offered to sell your car.

Private party price is the price that you expect to get if you are selling to an individual.

Retail price is the price that you expect to buy from dealer if you try to purchase a used car from a dealer.

The price should be based on the mileage, options, and overall condition of the car.

You can find price guidance from True Market Value report from Edmund's, the Kelley Blue Book, and the NADA

Official Used Car Guide.

Another way is to compare the same car with similar model, year, engine, type, mileage, and options at online classifieds

and see the lowest, average, and the highest price.