[jyl]

• You buy something and sell it for a lot more than you paid, because the seller didn't know what he had. Do you make it right by offering the seller part of the profit?

No. Caveat emptor goes both ways.

• How about this. You buy a desk at a yard sale, and when you get it home, you find an envelope with a huge stash of cash taped to the bottom of one of the drawers.
Keep the cash? Give it back? Give part of it back?

I'd like to think I'd give it back, but haven't been tested.

• Or, you buy a painting at goodwill, because you like the frame. When you take the dust cover off the back, you find a certifiable first draft of the Magna Carta, thought to be lost for centuries.
Who owns that valuable document? You? Goodwill? The state? Great Britain?

I own it.

• Maybe you're putting a sprinkler system in the backyard and you find a coffee can full of gold doubloons. Yours to keep?

Yup.