Even if we take what corporate says at face value (there was no agreement, or the agreement is null, or it's an agreement that the previous owners agreed to) that still just means that the store possesses property that they do not legally own. Whether or not they legally came to possess the sets seems irrelevant here.
I'm not a lawyer but I don't see how the Mansells ever stopped owning the lego sets.
Even if the consignment was undone, they don't get to just keep the collection. The agreement can almost certainly be terminated, but the collection would then be returned to Mansell.
The payment was never provided, so the ownership never transferred. It has always been with the original owner. This isnt complicated.
"How did you acquire these sets?"
"Uhm... don't know they just appeared out of nowhere"
Straight to jail.