I'm very sorry to hear of your loss. It really does sound like you've tried to do everything right.

It "could" have been IBD. I mean, it's possible but I wouldn't immediately suspect that, as mentioned, it's very rare.

Also, unfortunately when dealing with live animals, there is always the possibility that an animal just up and rolls over and dies. I've heard it from some of the very experienced breeders that an otherwise perfectly healthy and normal animal just goes downhill and dies.

Without a necropsy, it would be near impossible to know. Did you contact the seller? What did your vet say and do for the snake? Did the snake get any injections? MEds?