School buses have a weird used market, because there are different groups of people that want them for different reasons.
There's a huge export market for school buses to South America and other countries. There are a lot of places where $10,000 for a used US-grade school bus is a bargain - even held together with duck tape and bailing wire. (I mean, we're looking at using them and re-purposing them again - so the idea can't be all that crazy.)
Selling a bus overseas requires an exporter - and these exporters buy in bulk. They're some of the ones who will clean out dealers, or seem to swoop in and clean out some of the auction sites. It doesn't matter if they get a lemon or two, they'll make enough profit selling and exporting.
On top of that, you have some bus dealers, who also buy in bulk, and often sell to these exporters. Some of them are really just flipping buses for a profit, others are actual honest dealers that do service, and actually care about the product they're selling.
That's one of the reasons you see crazy prices on a lot of buses - sellers hope that there's someone out there who will pay it. (And there's a chance that there is.)
A lot of it is just people asking waaaay too much for what they have, because they don't know what they have - that they have a slow bus with a not particularly desirable transmission, or that they have a bus with a brake system no one makes parts for anymore. It's the same thing as any used car seller- their often favorably biased towards what they have.