Okay, so if you've ever bought any firearms through a dealer, then should the government find it necessary to come a looking, they already have a record that you own or have owned firearms, so you go on the check anyways list. Now if they come and they find you have a firearm(s) they don't know about, especially one that may have an adverse history, now the trouble starts. If they decide even just one of the guns in your collection is a bad apple, they will likely take the whole bunch, followed by a search warrant on the rest of your house, and finally cuffed and stuffed.

Unless your entire gun collection is off the books AND you have managed to keep your mouth shut about it all this time, I would say that buying as a private sale, especially from someone without a bona-fides, is just asking to be labeled a suspect.