The listing could be removed without the transaction getting cancelled.
For example, I ordered a $20 giftcard on eBay, paid through paypal, and received it in the mail quickly. The card was perfect, just as described and I made purchases with it. A couple days later I received emails stating that the listing was removed from eBay.
The seller had accidentally made an error in the listing that violated eBay's listing policies, probably "You can have only one gift card listing at a time, even if you have more than one eBay account."
The problem wasn't with the product, it was how it was listed, so I didn't have a problem with the transaction and didn't dispute it.
