OK fans, BG will tell you what's going on here.
This is absolutely 100% from a feeder rodent. Unfortunately this has happened to BG a couple times over the years....and probably almost every larger breeder. Always make sure you get well fed rodents. It is perfectly safe(99.99999% of the time) to leave a rodent in with a snake over night if it's well fed and watered, if not, this can happen.If you have hides in with your snakes you may miss the fact that it didn't eat and leave a rodent in for a few days. This can happen then also. Be careful fans.
This snake will probably be fine, but you don't want it. Maybe for half price. There is a good chance it's not a good feeder, and this may make it worse. Get your money back for this animal. You'd be foolish to take it unless it's at least half price.