Thank you both, I really needed to hear that!
It feels doubly unfair because both of my current snakes are rescues I got through a local reptile rescue I volunteer with, and I knew when I took them in there was a very good chance they wouldn't make it, so it's ironic that this happened to the one snake I actually bought for myself with the assumption he'd be healthy. It sucks to lose a dream snake so suddenly and miss out on what should have been many happy years with him, but I know I still have two awesome, thriving snakes and there will be another BRB for me sometime in the future.
I've already texted the seller to let him know what happened. I don't blame him at all, and totally agree this most likely wasn't anyone's fault, but I'd want to know if I was him, and I know he has a huge collection so I definitely don't want any of his other snakes getting sick if it was a case of something contagious. I'll also be trashing all of the boa's stuff and plan on totally disinfecting the room he was in just to be safe - he obviously never graduated from quarantine so there weren't any common supplies or anything between him and my other snakes, but I really don't want to risk anything getting passed on to them.