There's just no way of knowing what it could be Tanya. It could be that this baby snake was never properly hydrated after shipping, it could be that it's never taken a feeding or it being dominated by the other snakes so doesn't get much access to a water source. It could have a load of internal parasites.

Whatever it is, that store has a responsibility to address the issues, have the snake seen by a vet and house it appropriately so it can recover it's weight and health. It thoroughly disgusts me to see a pet store putting any animal that is obviously less than fit out on their sales floor for some poor unsuspecting newcomer to buy and then end up with all sorts of problems.