Probably, you could find someone on the forum with the item. The person does not have to share the item with you, just the hash. From the hash, you do not know the content so does not break copyright. However, you can know if the hash is different than what the seller's trying to sell.
Maybe you could save a serialized collection of checksums / digital signatures in a blockchain which would allow people to compare it to the products they download to ensure legitimacy?
There is a working service for that already.
It is not my intention to advertise any platform's solution now, but if you are truly interested, I can suggest an open-source example.