According to the Kelly Blue Book a 2000 F-350 diesel, 4 door, 6 ft bed in just average condition with just the basic options has a trade in value of $12K to $15K.
Dealer selling price according to KBB for a similar basic model but would currently have an average total miles of 168,000 and would be in need of repairs such as tune up, paint needs touch up and is faded, minor rust, minor dings and scratches, minor engine leaks, interior showing wear, 50% tire tread. Dealer price would be $15K to $18K. A truck with 1/2 that miles, more options, and better condition the price would jump considerably.