More news soon, says Microsoft
Xbox Live appears to be experiencing difficulties this morning, as per the official Status page - Marketplace transactions and Xbox forum usage are currently "limited". If you're thinking of buying anything via a PC, tablet or phone, you should probably hold off till Microsoft sends the all-clear.
"Are you unable to purchase content from the Marketplace?" reads an update. "Our team is engaged and diligently working toward a fix. We'd like to thank you for your patience during this process. Another update is coming your way in 30 minutes." That's from about 30 minutes ago - I'll edit this as and when there's anything else to pass on.
good old Microsoft