The company did not start out trading MtG cards. Mtgox's founder launched mtgox using a domain name that he once used for a MtG site. The founder then sold the company to Mark Karpeles, the current owner.
It's not. The original commenter insinuated that the site was somehow inferior do to a history of selling playing cards. As it's just the domain name that was reused, there's clearly no association at all. A name is a name is a name.
Not quite. They are two distinct business ventures linked by the operator and name. The name is irrelevant, and the operator clearly ran them as distinct entities.