Writing mere statements can not be a proof that what you claimed happened really happened but backing up your claims with facts can actually sustain your claims here. I have read through and there is no difference from what each and every one responder has said, OP for you claims to be really taken seriously, there must be some proof otherwise it will be a null and void scenario.
I believe this is a thing of speculation and on assumption basis for which OP is working with.
You can't prove it directly. He caught sellers and knows all the nuances.
Perhaps you can look at the ip he used previously and the browsers he used. Although, the previous owner could have transferred the VPN or said which one he used.