Go talk to a commercial loan officer at a bank. Somehow they can determine the value of a truck.
For example the last purchased made by the fleet was a KW. The owner wasn't sure of the true value of it and used truck paper, ebay and other sites to come up with a number. I went through the truck to find a number of issues to be fixed. I took all the info to one of the commercial loan managers at BoA and she did the work for me, found out that the truck was a bit over priced for a loan and when I figured the price of the repairs, I adjusted my offer which the owner agree with and the truck was purchased. The work paid off, the owner saved more than $15k.