You want to resolve this in a hasty if not aggresive fashion...read on:

Bob does this quite frequently, and I'm assuming he has to cash the cheques that people give him, and of course that you would have records of the cheque that you sent him.

So, give a little call to revenue canada. I guarantee you he's not paying the taxes he's supposed to when selling this stuff, and in fact, he probably doesn't even have a business license. They'll audit him, and he'll end up owing thousands.

Or he could just pay you.

Just a thought.