Are they really truly attempting to sell something illegal? Has this been verified?
NO ONE keeps my $ i.e. deposit, for something they are attempting to sell that is in fact, illegal.
First things first, I would be 100% certain of the illegal status of the item. Then write them a letter of dissatisfaction, cert. return receipt, stating the facts as outlined. With a caveat, return my 50% fee within five business days or the letter will also be mailed to my attorney, local LEO and state AG for determination.
If they haven't seen the downside at that point I would file suit against that person, other damages such as , loss of use of my personal property , the 50% fee, time trouble lawyers fees. Additionally, that is an industry EXTREME high amount! I've heard of 5%, 10, 15%. Which promotes discouragement of buying but not taking possession. The reason these are the %amounts are, they work. But 50% for...what? Why not 60%? 70%?
Etc.