To be pedantic, MtGox doesn't actually stand for "`Magic the Gathering` online exchange" like mherdeg was suggesting, but rather for "`Magic the Gathering online` exchange". A subtle difference,perhaps, but MtGox was never intended for trading cards like these (Alpha Black Lotuses and suchlike). Rather, it was intended for trading MTGO cards.
In other words, not an online exchange for Magic: The Gathering cards, but rather an exchange for Magic: The Gathering Online cards.
In other words, not an online exchange for Magic: The Gathering cards, but rather an exchange for Magic: The Gathering Online cards.