L4D2

Try to purchase it from an online browser through the Microsoft store & be sure you payment information is correct & up to date.

Unless you are from the USA, those links provided won't help.

Please go to http://marketplace.xbox.com/Product/Left-4-Dead-2/66acd000-77fe-1000-9115-d802454108d4 and check if the game
shows up there for your country.

Then try to buy it through the website.

A hard reset may solve this problem.

Hold down the power button until the system shuts down and unplug the power cable.

Wait for 5 min and plug it back in and restart the console

The title has not been delisted or removed from sale, at least in the US region.

It can be found here.

So in that case we want to check if it's that title or payment options associated with your account. You can check to see if you get a similar issues or error message when trying to make a purchase on another title. I would also recommend visiting your
Microsoft Account page under Payments & Billing > Payment Options and check that your methods are all correct, valid, and up-to-date.

Let us know if you run into other issues checking those items and we can help troubleshoot further.

Contact official support to see what has happened.

http://support.xbox.com/contact-us