is it wrong to withold a source?

I know its a little late, but from the other side of the fence (the guy with no sources of any kind) I still agree with everyone here. If you know someone is trustworthy then sharing sources is fine but there is no need to risk shooting yourself in the foot with a complete stranger.

If you want to help out a stranger, then offer to pick up a few sticks and sell them to him at the price you paid. He gets the sticks for a good price and you keep your sources safe. Everyone wins.