Good question to bring to the Discussion Board. We are fellow buyers and sellers, and pleased to offer advice.
If your item is past due and assuming you have no tracking, your first step is to Contact your Seller and ask for a tracking number. If none is forthcoming, don't panic because not everything is cost-effective to ship with tracking but you should hear a response from your seller at least to indicate that. You can find your seller under your Purchase History for your item.
The next step is to open an Item Not Received case in the ebay Resolution Centre. Don't accept an offer to replace, as it may not arrive in time for you to agree it has come before your open case times-out.
For more information on how you are covered under the ebay Moneyback Guarantee, please read this: http://pages.ebay.ca/ebay-money-back-guarantee/