I had a Sprag truck if I remember correctly (someone will correct me if I am wrong) from when I replaced a wheel cylinder on it, You should be able to check its operation if he will let you jack up a front wheel remember the spider gears in the front axle will allow you to turn the front driveline over to check the Sprag, by putting the trans it in gear and turn the wheel It should stop after some turning in one direction or the other, you must have the truck off, park brake on, transfer case in high or low, rears blocked and select a transmission gear as on the Sprag its controlled by the rod on the rear of the trans.
My
, if it is an m36 you probably will not see another for sale for a long time in your local area if ever, they come up on SS every once in a while there was 2 on here, so if the truck is straight I would not even be concerned with the transfer case type or condition except to use as a bargaining chip at that price range, if it’s a 36 inspect the frame it has kind of a double frame between the wheelbase, a slight a small amount of rust will be normal and a slight separation the bottom frame lips due to some rust is ok ( over the years these trucks sat outside the water gets in the top and rusts between the double frame) but watch out for major rust or rust through you will know when you look, you can check to see if the frame is weak or bent by sighting down the top of the frame from the rear of the truck mine is nice and straight, hope this helps, also I wouldn’t worry about rusted sheet metal it can be fixed and or body parts replaced but it would affect the price, might be why its only 3,500.
Hope this helps.