What did s/he actually cancel (a) your bid before the auction ended or (b) the transaction after it ended? Those are two different things with different consequences:
(a) It is specifically against the rules for a seller to cancel all bids and end the auction early without a winner IF it is because s/he deliberately set the Starting Price lower than s/he was willing to sell for, and it hasn't reached that high. This is considered fee avoidance because eBay charges a substantial fee for sellers to have a Reserve Price so they aren't promising to sell as low as their Starting Price. But there are legitimate reasons why a seller would have to do so, and a few years ago eBay got tired of having to divine the subjective intent of a seller who took that action, so it adopted some procedures that make it less attractive to do so with that intention: it may only be done before the countdown reaches 12 hours AND it charges a fee (the FVF% for that seller in that category times the highest cancelled bid) when s/he does so (with the first such fee per calendar year waived to allow for the very occasional legitimate reason for doing so).
(b) The seller already got a "defect" when s/he cancelled the transaction. 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--if the seller said that x was the reason say "seller said x. . ." so people don't think you are just jumping to conclusions) 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.
