Online sale, there is a paperwork trail. As to police trace, seems like effort is half hearted. LOL, I'm still waiting for a call from the police, to hear they recovered my LH Winchester 70 XTR, that was stolen from my apt in 1987.
A few years back, I bought a used rifle at a pawn shop, for my son, who said he might go hunting with me that year. When he changed his mind about hunting, I traded in the rifle at another gun shop for something I wanted. 2 days later I get a call from the police, saying a fellow recognised his rifle that was stolen earlier that year from him. It was the rifle I had traded in. I showed them the receipt from the other store, where I had gotten the rifle. That store owner stated he had checked the data base of stolen guns before accepting the rifle from another seller. Police did get the bad guy, but only after following the papertrail, AFTER the orignal owner saw his rifle at the gun shop, heehee.
As to Handi's, they can be modified with barrels, stocks, and some changed to shotshells. I can see how a thief might want to sell parts separately. But I don't think it would happen much at established Forums with posters having high postings and familiarity.
My understanding is that many folks leave a Handi-rifle with one certain barrel on it, once it has proven itself a super shooter. When I saw what people were willing to pay for Handi parts, I was tempted to part out my .270. But it shoots too darn good. I decided to keep my only Handi-rifle, when it didn't sell as a whole rifle. I only buy FTF from people I know. The new Marlin I bought, came from a retired AF officer.