eBay doesn't allow presales unless the seller will be shipping within 40 days of the date of purchase. https://www.ebay.com/help/policies/listing-policies/presale-policy?id=4252&st=3&pos=1&query=Presale%...
What was the estimated deliver date you were given when you paid (check your "Order Confirmation" email)? If you don't get a package from the seller by the time the estimated delivery date you were given when you paid has fully passed, go to your Purchase History for that item, click the link for more options and select that you haven't received the item, then follow instructions from there. If you haven't gotten the item as described or a full refund after 3 days (but before 21 days) from doing so go back and you should find a link to "Ask eBay to step in" to get your refund.
Do not let the seller string you along past those deadlines. If you come across a listing where the seller in the description says the shipping date will be more than 40 days away DO NOT BUY IT. It is very likely a scam. Check the estimated delivery time to your location in the Shipping, Payments and Returns tab of the full website and if the dates don't line up with what the seller is promising or what you know of when the manufacturer will be releasing the item, pass it up.
If you let the seller string you along past the 30 days (again, from the estimated delivery date eBay gave you at payment) deadline, you would need to go with your payment source to have any hope for a refund (PayPal allows 180 days from payment to file, credit cards will have their own deadlines).
