It's not possible for the seller to cancel a cancellation, but the buyer is usually asked to confirm that they didn't pay for it, or that they received a full refund (I don't know what happens if the buyer confirms in the negative, though, i.e. if the request is pending, they pay and then respond to the request saying that they did pay, presumably that would mean the transaction stands on eBay with no FVF credited back, but I don't know what else would happen, re: defects, refunds and so forth).

I recently cancelled a transaction at the buyer's request, but they never confirmed anything in response, then came back and bought other items and paid for everything in one payment, including the item that was pending cancellation (cancelled items remain in the buyer's cart, so sometimes they can mistakenly pay for it). They said they wanted it after all, so I posted off all items together, and then a few days later the transaction was "cancelled" by eBay because the request timed out without a response (but, that just means the FVF were credited to me, nothing else happened as far as I know).