What do you mean 'the buyer cancelled it'?
The seller (you) can send the buyer a Request for a Mutual Cancellation. But the buyer cannot unilaterally initiate a cancellation.
If YOU did not initiate the MC, or if YOU did not have eBay cancel the sale (because the winner was somewhere you do not want to ship, for example) the transaction is still open.
If the winning bidder has not paid, go to the Resolution Centre which is in the dropdown menu up on the right under Help and Contact.
Start an Unpaid Item Dispute.
If the bidder does not pay, the transaction is cancelled, you get your fees back,and you are free to either offer a Second Chance to one of your other bidders or to re-list. EBay will not relist for you.The choice is yours.
The deadbeat bidder gets a Strike for non=-payment. Many if not most sellers have an automatic Block **against bidders who have Strikes making it difficult for her to bid on eBay. Multiple Strikes will get her booted off the site.
Do you mean a bidder cancelled her bid? No biggie. Better that than chasing an unwilling winner who will never pay. But in that case one of the other bidders would have won.
Second Chances are free. But if any time has passed, like 48 hours, your other bidders have moved on.
In future, go first to the automated system. Trying to get a human being is difficult and while the reps try, you still will be referred to the automated system, which handles thousands* of disputes daily very efficiently.
*Don't panic. At any given moment, eBay has some 800 MILLION listings. Disputes really are rare.
** Strikes are much more efficient in dealing with deadbeats. Most sellers only learn who their customers are after purchase, because most sellers use Fixed Price not Auction.
