Auctions are the best place to get the best deal on a used bus. Most come from being recently retired due to age or mileage, but are well maintained by the school system. Often you can call the transportation garage and speak to the manager who can give you more details on the bus. They have no dog in the fight themselves, so they are usually honest about any issues. Many of the bus sales outfits buy the same buses , wash them, double/triple the price and park it on the lot. Look for buses at auction in areas that are not near the rust belt to minimize any rust issues that WILL be present.Yes, you take a chance bidding/buying something unseen, but I've gotten excellent deals on buses bought sight unseen from 2500 miles away. Assume any mileage stated is incorrect unless it shows approx 10k miles a year.
Don't look for a bus anywhere near your local area, you will be disappointed no matter how cheaply you can get it.
NEVER buy a bus unseen is a sure way to pass up some awesome deals.