The succinct answer as to why everybody who bought that card has the same MAC address is because the company who made it screwed up. Bad. I've heard of companies occasionally created a duplicate or two, or screwing up a batch of cards, but all of them? That's just horrible. Don't buy anything else from them.
MAC addresses are supposed to be unique, yes, and can sometimes be changed by flashing the NIC with a new one, but in this case, I don't see anyone who's figured out the procedure to do so for that particular card. It's supposed to be burned into ROM, so it's not a trivial procedure to do, generally.
So your best best is to use the command @Nils gave you on your Linux box, and if you put in a Windows machine, you'd want to change the
Locally Administered Address, which you can find under the
Client for Microsoft Networks.