Looks like Xbox Marketplace is experiencing a few technical issues at the moment. Issues like not being able to access the site or buy anything.
We asked Microsoft what's happening and this is what they said:
For the past few hours some users may have experienced some difficulty getting into Marketplace or making purchases. Microsoft is actively working on a solution and expects to find a fix soon. We apologise for any inconvenience this might cause.
As the site being down means Microsoft seeing fewer money points being spent, we expect they'll fix it pretty sharpish.