If you go to your Purchase History, you will see Order Details which gives you a delivery estimate. Under the ebay Moneyback Guarantee, you have 30 days after the last estimated delivery date to make a claim through ebay. Open an Item Not received case in the Resolution Centre. It should be as simply as choosing one of the options in a drop-down menu. As far as etiquette is concerned, you should ask your seller for shipping details first. If you have done that and received no response after about 48 hours, go straight to the Resolution Centre.
If that 30-day timeline has passed, you still have Paypal which allows claims up to 180 days after purchase. Same process, but find the transaction in your Account Activity. Ditto for your credit card company if that's how you funded the purchase instead of using paypal.
Do NOT accept an offer of a replacement from the seller. This is often (although not always) a stall tactic designed either to time-out your case, or to trick you into closing the claim before you have an item or your money back. The only time I would accept an offer of a replacement from a seller would be if I had a previous relationship with them and knew them to be trustworthy, and also that they could deliver the good within about a week of time. Any longer and you start running out of time on your claim.
After you get your refund, you can always buy another item from the same seller if you really have your heart set on that thing from them. You are protected all over again with a new purchase.
Come back if you have trouble with it.
