The problems I see are:
Most of the dark web sites are not publically known/accessible.
I would think the number of guns that are posted with sn in the pic would be very few.
From reading, most of the stuff sold in this black market is information or large batches of drugs. There have been a few cases of mail order home delivery sellers of drugs, but I have a feeling they do not last long. Too easy to get caught shipping or receiving.
I read that a very popular way to move stolen goods, including guns, is that different groups travel to meet and do a criminal swap meet deal. Then take the items back to their area, usually a different state, for sale. That means unless the report makes it above state level, it never gets seen in the other area. Also, unless it is a distinct or rare item, they can sell it on a classifieds site to an individual and the sn never gets checked.