Did the seller give you a tracking number? If not Contact him and ask for the date of shipping, the service used and the tracking number.
Don't worry if there is no tracking number. That is a Seller Protection and does you no good at all, except to upset you.
Don't get into a conversation.
You want a number or a refund.
Do NOT accept an offer of a replacement. It won't arrive either.
If the seller is not helpful (helpful means a refund or a working tracking number) open an Item Not Received Dispute in the Resolution Centre at the bottom of this page.
Did you buy from a Seller in the USA or UK who used the Global Shipping Program?
If so, the tracking is weird, but you will get your item.
More info if you need it, just Reply to this thread.