If they KNOW the items are stolen, they have a victim. The items have almost certainly been identified as being the property of another through serial numbers, description, or identification by the victim.
The proof is really not all that hard. Now, the defendant needs to show that he had no idea all this property was stolen ... usually we find that the story involves some mystery person that gave it to them or sold it for a ridiculously low price.
Since we have no idea what info the police have, we can't say how strong the case is. But, I have found that we usually have a good case for possession of stolen property when we find multiple items int he possession of the same person .. it's called "a clue."
- Carl