1) The only way you could get your money back is by suing them, unfortunately--that's the only way to recover money from another person if they will not voluntarily refund it to you.  It is not likely cost effective to sue for this2) It's also not clear you could win a lawsuit--not being satisfied with a purchase is not a good reason to get your money back, and if they violated someone's trademark rights, that's not your problem or grounds to sue--it's for the rights holder.3) You could inform the rights holder of what was done if you wish; the rights holder can then decide to take action if they want.4) You are not in trouble for buying the shirt: you are not the one who created or sold it, violating trademark rights.