Unless you got a refund the seller hasn't cancelled yet (eBay automatically refunds on cancellation). S/he might be trying to get you to ask to cancel (do NOT send more money) so s/he can claim cancellation at buyer's request (which lie keeps him/her from getting a "defect". I'd respond with "I expect you to honor the terms of the sale and ship promptly for the price and s/h fee you set when you set up the listing which I have already paid."
Then if s/he refunds (or already has):
The seller already got a "defect" when s/he cancelled the transaction (unless s/he lied to eBay to blame you, in which case we'd need details especially what eBay said the reason was in the cancellation notice). That is the primary tool eBay uses to weed out bad sellers, downgrading the selling status of those who get too many relative to their sales volume (as opposed to having to pay employees to actually investigate to see how likely it was that the seller had a legitimate reason to cancel vs. deliberate policy violations).
You may leave appropriate (calm, factual--quote the seller on the reason in quotes and be sure to mention asking for more money) feedback to warn future buyers/bidders. If the usual links have vanished, go to anyone's Feedback Profile (doesn't matter who, click on the feedback score in parentheses behind the username) then scroll down to below the last comment on that page to find a "Leave feedback" link that doesn't vanish (it brings you to a list of items you can leave feedback for).
Theoretically you could sue the seller for breach of the contract of sale (for the difference between the auction price and what you ended up having to pay for a replacement), but that is almost never practical.
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. Come back for for help with feedback since there are some pitfalls involved when there has been a case.
