Good chance the serial numbers, etc end up filed off since they will almost certainly become crime guns. Since the serial numbers are known, you can't really sell them to collectors/enthusiasts because eventually one of those will turn up somewhere and the trail could be followed back to the people who stole it.They're not criminals because they're smart. If this large amount of pistols are stolen, I doubt that person(s) is just going to hide them in their closet. Sooner or later they will sell one on the street, try to pawn it, or sell it at a gun show. One will turn up at a crime scene or in a stolen vehicle. A snitch will hear something. Then it begins to unravel. But I'm just speculating.
In the criminal world a gun sells for a lot more than it would when sold legitimately in a shop. This is why you'll never see actual criminals surrender their guns at "buybacks" even if they offer $200 gift cards in exchange for a Bryco that isn't worth $80.