This is from the FAQ:
In the event that you have to file a Help Desk Ticket due to a trade gone bad, or not receiving the cards you were promised, The Mod team will do everything it can to help you resolve the problem.
In the event of a trade gone bad we ask you to wait at least 3 weeks from the posting of the trade before filing a help desk ticket, mail can sometimes be slow.
If you need to file a help desk ticket for a trade gone bad, the help desk ticket MUST be filed no more than 8 weeks after the trade was posted. This may seem unfair but we feel that is an appropriate window for trade gone bad resolutions, any longer and it becomes extremely difficult to find tracking numbers, receipts, etc.
The Mod team will not assist on anything if one of the following is true:
You made a deal on the site but did not post the trade in The Trade Manger
You Purchased cards from a member who has less than 20 Bench Points
You made a trade with a person who has the MSF (must send first) Avatar, but did not wait to receive their end first
You purchased a card from a trader and paid via Paypal "Gift". Abusing Paypal's "gift" option leaves you no recourse to reclaim your money in the event that you do not receive the item(s) you paid for. If you pay with Paypal "Gift" you are doing so at your own risk
At the bottom of the page of that trade in the Trade Manager there is a report trade button. Please report it there.