The KBB for that truck, using your zip code (62812) is a little over $15,300 SR, and your truck, this estimate is more rough, is worth about $2,000 trade-in in "fair" condition (minor mechanical problems)
Please, I mean you no offence, but a lot of used car lots make ends meet by charging unknowing people outrageous prices. Either way you're getting shafted. Get a printout from KBB and put it to his face, and he will drop down to KBB value rather than lose a sale. I'd get a mechanic friend to come loo kit over for you, too, just to be safe.
S-Mart Corporate Security Division
Necronomicon Retrieval Specialist