Have not received the check back from seller and Gunbroker has not responded

I had sent the seller Frank Matusek a bank Cashiers Check for an item I bought from him on March 23, 2026.  He contacted me and said he had lost the item and would send my cashiers check back.   He lives in Illinois and I live in Indiana.  To this day, April 17th, I’ve not received the cashiers check.  I submitted a ticket to Gunbrokers on Sunday, April 12th.  I haven’t received any information from Gunbroker.  When I have emailed the seller, the email comes back not able to deliver.  Any other ideas on what I can do?   Thank you