First, good for you. How these idiots still run this scam is amazing.

your response should have been to the guy in the van - Sir, you are either a lier or a thief, hmm perhaps both. Which is it?
to the victim - if its stolen merchandise as he is claiming, than you are committing a crime as well.

then you should simply call the police in front of them and report them both while giving a description of them and their license plate numbers.