Where have you NOT heard that petstores are a HUGE NO-NO?
Pet places want to sell their snakes. That's it. Not take great care of them.
IF you get eggs, they're most likely not hatch. If they hatch, they're most likely not going to survive.
IF your Casper had RI, there's a pretty high chance it'll get to your new one.
Your new one might have mites or something else, which, in that case, WILL infect your other snakes.
Even if your snakes will keep eating, they're not gonna be HAPPY about it.
Good luck.