I owned a small engine repair shop for a brief time. (Not brief enough.) I wouldn't be surprised if 80% of my business was the result of abuse or neglect. The same goes for autos. Trannys get neglected badly. Why? Because people don't check the fluid often enough and don't service them. Trannys dislike heat, old oil, and a lack of oil. A tranny cooler will extend the life, but if it didn't doesn't have one, that is not good, especially for a work vehicle. The reason people don't service is because people tend to take poor care of their vehicles and trannys are expensive to service. Changing tranny fluid is a messy deal most of the time, so that also discourages maintenance. Since it was clearly used hard, it should've been serviced about every 25,000 miles or so. If he has receipts showing that and for other work he has had done, then that would help out a lot.
Many people drive vehicles hard, too. They don't stop before shifting gears. They drive too fast too soon. They shift hard. You know how most people won't take care of your stuff like you do? Same goes for vans.
It's a one ton, so you know it has the beefy tranny.
I like to see receipts of all the stuff that has been done over the years. This is rare, but I'm more willing to buy and to pay a good price for a stack of receipts. If it was for a business, the owner should have the receipts for at least 3 years- long enough to establish a pattern of neglect or maintenance. The lack of receipts would be more indicative to me than the inside. I'm ok with a vehicle being used hard, but it must have been well cared for.