You can probably go back only so far, though. "I bought it from two guys at a comic shop in Queens, who I did not know. I don't know where they got the art from. I have no idea whether they stole it, or someone with authority at Marvel gave it to them." And, sadly, a lot of the people you'd have to depose at Marvel are dead, or very old and you'd have a very small window of opportunity to obtain this information.
So, I suspect that a LOT of this art will start coming out of the woodwork once the current owners pass on, and their heirs liquidate it.