Ok, so let's talk about an entirely hypothetical situation. Let's say that I (hypothetically) play Magic the Gathering. Let us also say that I (hypothetically) have a friend who does not play Magic the Gathering. Let us then say that he has a brother who used to play Magic the Gathering, but no longer does. Let us say that I (hypothetically) visit this friend, and, whilst mucking about in the basement, find a box of cards. Let us then say that after finding the brother, we all adjourn back in the basement and begin digging through the cards for fun. Let us say that he says I can have any and all of the cards if I so wish. Let us then say that I find a Black Lotus.
A perfect condition Black Lotus.
For those of you uneducated in the fine game of MtG, This is what a Black Lotus is. [http://sales.starcitygames.com/cardsearch.php?singlesearch=Black%20Lotus]
Now, I (hypothetically) have 2 options. One, I can take it and make easy money. Or two, I can tell him how valuable it is and let him sell it. Now, 5 grand would come in handy for my family. But it would be just as nice for his.
So, instead of thinking it out and taking responsibility for my actions, I'ma let you decide. Do I tell or sell?
Hypothetically.
EDIT: Note that no matter what anyone says, I'll probably (hypothetically, of course) tell him. I just wanted to see what other people would do.