While it's unlikely anybody is going to give you legal advice here there may be things you can do depending on how you would answer a couple questions.
First, did they mark the item as shipped and enter a tracking number? The seller doesn't get paid until the item is marked as shipped. I'm not sure how to process an order cancellation, but that option might work for you.
Second, did you pay with PayPal? PayPal has a resolution system that might also be a good option.
Finally, check to make sure your contact information is up-to-date. There is a chance that the seller has tried to contact you but is unable due to incorrect contact info. It's not common, but I've experienced this on other sites.
I've been doing online order fulfillment for years, and the biggest thing I've learned is to always answer order inquiry's QUICKLY! Just acknowledging a buyer's concern can make solving problems very easy. I'm sorry your seller has gone silent on you. Good luck resolving this very frustrating situation.