I buy and sell on eBay a lot, particularly MTG cards.
If the seller cancels, not much you can do.
As general advice, I do strongly recommend never pre-order something on eBay that won't be shipped to you within 30 days. Chandra should be o.k given how soon Kaladesh is coming out, but some people pre-order cases / boxes / singles far ahead of time, and that is very risky. When you buy something, eBay will create an expected delivery date based on the purchase date, handling time, and distance it has to travel. If you don't file an issue within 30 days of the expected delivery date, you lose all rights to buyer protection on eBay. If you pre-order something that is coming out in a month or two and the buyer never sends it, even though they put in the auction that it won't ship until "x/y/z", eBay will not back you. Paypal will, if you've paid with them and it is within their buyer protection period - but you need to be careful with the dates.