Well, in my view this is a more serious offense, Not that the cards didn't have value, but when someone cashes a check and does not deliver promised goods, this is in my mind a more serious and tangible form of mail fraud, and likely will be viewed as more substantial by the local sheriff's office. The sheriff may not view pokemon cards as lost value to worry about, but cashing a check for $30 and then not following through on promised deliver of goods is easy for anyone to see as mail fraud.
I'd recommend that the Gym help all those that were defrauded by this trader to summarize the deals in a single letter, including PMs, mail fraud filing reports with the USPS (everyone should file these:
https://postalinspectors.uspis.gov/forms/MailFraudComplaint.aspx I think, and print them if possible, just to support the letter) and other available documentation, (INCLUDING THAT CASHED CHECK!), along with lists of the names and addresses of the people defrauded, and send it to the local sheriff's dept. with the bad traders real name and address and a single point of contact from the Gym. That should turn up the heat.
Just my two cents.
:nonono: OK just watch how you talk about us ducks. 
