The police do not routinely check the career history of alleged victims of crime; that is my experience from being a victim of serious crime and from supporting people who have been victims of crime, and I checked it this evening with a serving police officer. Realistically, the only way they would know would be if you have told them at some point.
So, let's say you rock up to the boot fair and demand that the stallholder returns the allegedly stolen goods; the stallholder says "Mate, I bought these from a guy I met in the pub." At that point, how are you going to prove that they are your allegedly stolen items? Do they have something distinctive about them that you can describe to the organisers before getting anywhere near the stall holder? If you have not, you are going to be out of luck.