I had a similar situation happen to me about ten years ago. I was perusing Gunbroker looking at 1911s like I often do, when I saw a very familiar-looking image in the thumbnails. I clicked on the auction, and lo and behold there was one of my own pistols listed on the auction. There were two pictures, full left and right views that I had shared on the Internet forums which this scumbag was using to create a fake auction, hoping to dupe somebody into winning it and sending him money. I took a quick look at the seller's other auctions and found that he had done this with several other pictures of guns he'd plucked off the Internet as well, even stealing some images from a well-known collectible firearms website. I notified the GB staff as well as the other site, and this guy was quickly shut down. From that point on I decided never to show the complete SN of one of my firearms unless I was actually selling it. What could have happened? Well, I could likely have received an angry e-mail from the would-be buyer had he later seen the legit pics of my pistol on the forums and assumed I was the one who screwed him. Then I'd have to go about explaining to him that I wasn't the one who created the auction in the first place. Of course he'd have to prove that it was me, and GB would likely back me up, but it still wouldn't be a whole lot of fun for everyone involved.
While I understand the point JohnnyP makes that full disclosure equals better help with information, human nature tends to breed opportunists who have no qualms about screwing some anonymous person out of their money. And like big_gus says, there's always a small chance that you really do have a gun that was previously reported stolen. When I have a question that requires disclosing the entire serial number I prefer to PM a few trusted individuals rather than telling the entire Internet that I now have Colt serial number 123456. If others don't care about the risks, well it's their choice.