I respectfully disagree with turf's summary. It may be that there are some jurisdictions where what's written is all true, but there are jurisdictions where some of it isn't.
The mere fact that the federal vehicle ID tag says "bus" doesn't automatically invoke all those headaches. For example my "other car," a Nissan NV3500 passenger van, is clearly marked "BUS" on that tag yet there's no need for a DOT number, log books, etc. This is because my family uses it privately. If I were operating a shuttle service then those things could come into play.
Some US states (I'm intentionally avoiding words on the spectrum of "a few" to "most" because I don't have data to support such a choice) allow drivers with a normal passenger car type license, called class D in my state, to drive pretty much anything so long as it's for their private use. As soon as a person starts using the vehicle in commerce by carrying objects or people for hire then commercial driver and vehicle licensing, logging, and other requirements are activated.
A notable exception is carrying people: whether for hire or not, a passenger endorsement may be required simply because a private bus is capable of containing more than 16 people. No big deal; a passenger endorsement is easy to add on a driver license and costs only a dollar or two per year.