As long as the seller didn't lie and blame you there is no impact to your buying status (even if you are entirely at fault and malicious you can't get negative feedback as a buyer so will always have 100% as long as you have received at least 1 feedback in the past 12 months). That's assuming the seller actually cancelled the transaction as opposed to simply telling you s/he had. If there is still a Pay Now link you need to pay within 4 days to keep the seller from blaming you for not paying.
The seller already got a "defect" if s/he actually cancelled the transaction for "out of stock" or the like. 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.
