I think this would be better placed in Apolegetics, or Liberal Philosophy or somewhere else.
The Mark of the Beast is not anything we would know until it's too late, despite all those bad Cloud Ten flicks. You've also got the buying stuff bit backwards: in the end time, you'll need the mark to buy stuff. The mark of the beast (according to various interpretations), can represent Roman citizenship (or more fairly, ancient Roman patriotism), a microchip (uhh, yeah, right), or a literal tattoo of the number "666". When reading Revelation, don't just read it literally, John put a bunch of hidden meanings in there. One thing to think about is "Babylon=Rome". There are plenty of interpretations out there, and none have proven any more correct than the others, despite what half-crazed street preachers and (again) bad Cloud Ten flicks say. Though, interestingly, in the Greek and Hebrew alphabets, have number assigned to thier letters. The name of the Roman emporer at the time Revelation was written, when all the letters are added up (in Greek), come out to 666.
At any rate, try Wikipedia, or about.com, don't do generic websearches, you'll get a bunch of obviously false interpretations saying that Catholics/Jews/Muslims/Freemasons are working for the AntiChrist (and if you've had a hard time reading Revelation, don't let demonic lies like that get the better of you), or that the Mark of the Beast is the UPC on products you buy, or some other stupidity.
Here's a discussion on this subject from another site, this should cover things better.
Christian Guitar Theology Forums
*Mutters to self, ignore the following* (why do people keep posting end times stuff in Philosophy? They need to make a separate section called "end times" in big letters... And why does every one say that revelations is hard to read? It's loaded with metaphors, imagery, and allegory, not stuff you interpret literally.)