I've bought a number of M1009s and M1008s, some listed as running, some not, some unknown. Some were previewed and some not. The only one we couldn't drive home or onto the trailer was listed as non-running. It had two bad batteries in it. We dropped batteries in it and it fired right up but had a bad transmission. I limped at 20-25 MPH about 20 miles to a friend's shop and came back with a trailer to get it later. Fortunately I had a very rusty but low mileage $650 parts truck to get a transmission from.
Some locations will let you start them during an inspection, some won't. Some locations will let you do a little work before removing them, some won't let you any work anywhere on the installation. I've even picked up trucks that were in motor pools and have had shop chiefs send mechanics over to be sure the vehicles ran and to fix any minor faults. It seems like every location and pick-up is different.
My advice to new MV buyers is generally that unless you have a good basic mechanical skills and tools and have a mentor to help you through the process you might be better off buying the first one from a private owner. That's especially true if the purchase is any kind of a financial stretch since with a private owner you can spend a lot more time inspecting the truck, and asking questions. Lastly, with a private purchase you'll be able to test drive it which is something you won't be able to do with GL.
If, like many of us, you are on a budget for your purchase I would advise that you spend no more than 3/4 of your budget on the truck and getting it home. These things are roughly 25-30 years old and there will be a lot of rubber and electrical things that need replaced, cleaned etc even if nothing major is bad.
Good luck with whatever you do.
Lance
