The only thing that is not illegal, that could be implied by your comment, is the sale of a firearm from one private citizen to another without the aid of an FFL. Personally, I think anyone who would make such a sale without the aid of an FFL is an idiot, as it could lead to legal complications in the future.
I've sold guns that way, but then I've always bought my guns without the use of a FFL, so the sale would *never* be traced to me. Well, theoretically, the person I sold it to could try to identify me, but that would fail, so the gun check would turn up someone in the '80s that bought it from a FFL, and got rid of it. That person is probably dead anyway. So all the tracking will find a dead end that leads nowhere. Not sure how that'd get anyone in trouble, unless you think the types of people to sell guns second-hand without a license would always buy it from an FFL themselves?
If I owned a gun and sold it to a private citizen, I would want to transfer through an FFL strictly so that I have documented proof that I transferred ownership. The reason being that I would not want the gun traced back to me if some sort of crime were committed with it. In the case that you mention, where you bought the gun from a private citizen, I see no reason not to sell it without the help of an FFL from a legal standpoint. Does an FFL perform a criminal background check when you do a private party transfer? If so, I might still want to do it for that reason.