Vid, I just read thru this thread - I'm not sure what else you need to see here - you received a knife back you do not stock from a different manufacturer than the one you sent out, he has not CALLED YOU, he found a suitable knife for $450 at a flea market, which can be done, but you need to work at it and at the RIGHT flea market, and now the replacement knife has been disposed of.
He claims to have now spent $450 locally on a knife in easy fashion, and from the sound of things, it would be exceptionally difficult for him to have found this knife locally.
That he may have spent anything locally is a red herring. That he is sending you back a knife you do not offer or sell is evidential.
I would specifically call your bank and card processor and inform them of the facts of this case, including the charge number, and tell them you believe this to be a case of fraud. I would then contact him and tell him that you are contacting the police about the incident and then do so, again giving full disclosure.
Once this occurs, they will handle the load and he can begin to answer some more diffucult questions, particularly about finding a knife which appears to be considered illegal in his jurisdiction, as well as what the disposition of this knife is, and how he managed to conduct what may be construed as an illegal lottery. If the organization does not exist, that may also be of interest to the local authorities since they seem to frown on invention of non-profits for whatever reason.