In most cases, learn your lesson and do nothing.
In theory, you could file federal mail fraud charges.
At the very least, report the trader to
www.gabtraders.com so that people can check the database on good and bad internet traders and future people can avoid your mistake.
Keep in mind international shipping times can take 4 to 6 weeks or more in some cases. Also, there are a lot of international freighters that are delayed or behind due to the disaster in Japan.
There's always the case that the cards did legitimately get lost in the mail. In some cases, ask the trader if the cards had been sent or if there is any way you can get your cards back (or some other reimbursement).
-wtk