Yeah, that's the most frustrating thing about this whole situation - it really was a case of me feeling great about him one day (and why wouldn't I? Perfect shed, no abnormal behavior, everything in the enclosure seemed locked in, and I figured he'd eat for me with time) and then finding him gone the next. I've tried researching to see if I can find any information about what might have happened, but it's difficult because, aside from him not eating, I didn't notice a single thing wrong. He didn't overheat, he definitely wasn't dehydrated, I checked him over for any obvious injuries...I'm at a loss.
I'm still waiting to hear from the seller, but I'm hoping he might have some clue as to what went wrong. I obviously don't have experience with BRBs, so maybe there was some sign of him becoming sick that I overlooked. Or he might have noticed something in his collection; I know he goes to expos and has a job working with reptiles, so that's definitely a possibility.
When I do try again with a BRB in the future, I think I'm going to seek out a yearling, just to ease my mind a bit. I'm looking forward to the day I'm able to post a success story here. And I can't say how much I appreciate you all - lots of people in my life don't understand getting upset over the loss of a snake.