In general, if it's not delivered to your address, you need to contact the seller to ask if the tracking number is right. The seller may give you a tracking number of another customer. If the number is right, he may filled the address by mistake. If all are right, you need to contact USPS to reflect the problem and check it for you as soon as possible. You need to require the seller give you a solution (refund you or send one again to you) if you couldn't get it at last.
In general, if it's not delivered to your address, you need to contact the seller to ask if the tracking number is right. The seller may give you a tracking number of another customer. If the number is right, he may filled the address by mistake. If all are right, you need to contact USPS to reflect the problem and check it for you as soon as possible. You need to require the seller give you a solution (refund you or send one again to you) if you couldn't get it at last.