> If not my family, then maybe I could go Robin Hood, steal from the rich and sell cheaply to the poor.
Note "for value". If the price is unrealistically cheap, the courts can point out that the buyer should have been suspicious.
> I can't imagine that it works that way. I can believe that as a buyer of stolen good without knowing they were stolen, you won't be punished. But you will still have to return the stuff.
You don't have to return it, it's yours now. The point is that the bona fide purchaser is not a criminal and acted in good faith. If, however, there is the slightest whiff of a hint of a suspicion, those bona fides collapse and title reverts to the original owner.
It is harder to prove bona fides than you might think. Got a great price? Not for value, test fails. Knew the seller? Not bona fides, test fails. Bought it from someone who has no history of selling those items? Possible notice, test fails. And so on.
Look, here's the thing. Lawyers and judges have been working on this system (the common law) for nearly a thousand years. For day-to-day stuff like property, the loopholes are well and truly closed and have been for hundreds of years.
That I was an inadequate law student and now am an inadequate explainer does not change the fact that the common law is pretty damn adequate at providing sensible legal protection for almost all imaginable transactions.
(IANAL, TINLA)