There are companies out there that will go out and inspect a vehicle for you for a fee.
I have contacted a couple and found that the inspections don't really do us much good.
They will tell you about body damage, interior condition, broken glass, tire tread depth, fluid levels etc.
They tell you little to nothing about the running condition.
I have purchased three buses. I got to inspect one of them before the sale. I wound up with three nice buses.
With the auctions you are talking a bit of a gamble. School districts are generally much more forthcoming and honest about condition that your local used car dealer. Sometimes service records are available.
When I bought my Bluebird it had been in route service until just recently when it suffered some body damage. I talked to the bus barn at the district and was told that it was a great bus but was just a couple of years from being "aged out" so they didn't want to spend the $ on the body work.
I felt pretty good about the condition so I bid on it and won. I showed up to pick it up without ever having seen it. When we set out for home I didn't know if the bus would take me 300 feet or 300 miles.
As it worked out, the bus is in great condition. Drives like a dream and make a great moving truck
If you shop dealers or Craigslist for a bus you will most likely be able to test drive it and take it to a mechanic for inspection. This has value. You will likely pay 2-4 times as much than you would at auction.
Remember, that guy on Craigslist offering you an $8000 bus probably bought it a few weeks ago, at auction, for a fraction of what they are selling it for.