Also, no one can easily sell/trade someone 400 microsoft points, since they're not sold retail in 400 increments and you can't gift someone your already-owned points. 41 people have peeked inside, we just can't really help. You seem to be in the same boat as many other CAGs interested in the new L4D2 DLC.
Your best bet is to buy 400 points directly from Microsoft for $4.99, just be sure to delete the credit card info from your XBL account immediately after you purchase them...