I wouldn't think the used barrel would be worth enough money to be worth the effort. In this case I cannot think of any downside to someone having the barrel with the SN on it, but someone else might think of something.
There are some potential downsides to posting pics of your guns and SN online. There have been guys who downloaded the pics and then posted the gun for sale. Of course when someone tries to buy it the gun is never sent to the buyer. When the buyer reports the scam and has the SN you might have some explaining to do later. I'd think it would be easy enough to straighten out, but still a potential issue.
Another possibility is an insurance scam. There have been people whose homes were broken into and they had 1-2 guns stolen. But when they filed the police report they will claim several other guns as stolen to get a bigger insurance check. I know for a fact that this has happened.
The others will usually be some guns they used to own but sold years earlier. The odds are next to zero that those guns will ever turn up since the only time LE runs numbers on guns in most places is if they are used in a crime. While I've never known of it happening someone who has a description of your gun and a matching SN could potentially list it on the report.
Sure, they may get caught. But people do stupid stuff all the time and get away with it.
