3. If you deposit money on it, then that inadvertantly implies that you are the one with the live card, see part 1 of my comment, so at least in some of the cases any previous owner would not be able to just get to the money you deposited.
You might have the live card but the owner does not need the card to use it online; he just needs the numbers on the card, which could have been prerecorded before selling the card to you.