How big is the bus? In feet.
Many people just risk it. Also if it's under 16000 lbs in most states you don't need a CDL.
Some bare buses could be under that, or slightly over it. My 8 window bus was under 16k with seats. it was like 15.5k, and when I removed the seats and demo it was 13.6k at the lightest. I'll be over 16k when I build walls and furnish it so I'll need one eventually.
If it has air brakes and you do decide to risk it, I'd at minimum advise reviewing youtube on how to do a pump down test and ensure your bus brakes functions properly. And maybe also review how to inspect air brakes to ensure they are functioning. If determined to be good you at least took safety into consideration.
The trip from Boise to WA is a lonely and quiet one, and aren't likely to run into anyone to pull you over as long as you are driving slow which in a bus is pretty much a guarantee. Additionally even if you are pulled over, you could get lucky if you show the officer the bill of sale and just tell him you're just transporting it from A to B for the purchase and plan to get a CDL if he even asks for one. Some won't. Ensure you have a Bill of Sale during transaction.
But I leave that choice to you.