A carbon seatpost breaking can leave a razor sharp spear to get impaled on, I went to a bike shop with a similar worry and the owner told me a horror story about a guy he knew. I tuned out the injury but it was horrible, perforated his anus or stomach and needed surgery. Your ears are your friend and I think you should trust them, and they told you something was wrong.
as for who is responsible/can you get your money back? Honestly I don’t think it really matters, you’re safe and it’s not really anyone’s fault. That said you probably have a case and if it’s a big store you’re justified imo.