You have hit on one of the most infuriating aspects of shopping on ebay, but your sellers knows the ropes, so-to-speak, and has correctly informed you of your choices. When an item is 'not as described', you can open a claim but you will be required to return it at your own expense with Delivery Confirmation. If you simply mail it back 'frugal post', the seller might say "Item? What item?" That amount will not be refunded to you, only the item price plus original shipping fee. Here is the link to tell you all about it, - scroll down to section 4:
http://pages.ebay.ca/help/buy/item-not-received.html
We have to understand their point, after all, in theory a seller has only your word that the item is defective. When you return an item, you get your money back, the seller gets the item back, - you both lose a shipping fee. It's the fairest system possible, really.
The dilemma you have is wondering if a second item will be sent, or if the seller will claim to have mailed another but mailed none, - at which point you have been delayed past the 45 day deadline to file a claim. You do not say what the item is or where it is from, but your decision may be based on the return fee vs. refund amount.
Whatever you decide, you can always leave feedback. You have 60 days to do that. The more civilly you can express the facts of the matter, the more likely you are to be believed.
Best of luck. 🙂
