Uh, by definition, firsthand means that YOU'RE the duper of them, or was part of the group that was duping them AS it happened.
I don't think that's what you mean.
BTW, a lot of Valorite hammers came from the Blackrock traders - specifically from the blackrock pieces that resulted from the ??? getting blown up (IIRC in Terethan keep?). In most cases, it was 1 or 2 people that got the blackrock from that initial explosion (because no one knew at the time how big blackrock would be over the next 2 years). IF the blackrock pieces that blew across all the lands also gave such things (they had a lesser chance - mine only gave ASH 30s and Agapites), and someone made a concerted effort to get them, it is very likely someone could end up with 10 or more Valorite hammers. What they did with them AFTER the fact is another matter.
And, make sure there aren't any fakes mixed in there as well (hammers made of Valorite, and used down to 15 uses)
Note that I'm not saying they aren't duping, but it's possible that someone could get a decent number that really aren't selling (have someone take off the vendor for a day to give the appereance), and are just sitting there (or moved around to make it look like there was a restock) - or they could be selling a lot of them, and have a large pool of them to dupe that would only be 1st generation dupes, not dupes of dupes of upes of dupes, etc. They'd have a lot more item ID tags to work with, making it harder to spot than, say, 20 hammers that have internal numbers indicating they are all dupes of the same hammer (which the GMs could easily spot).