You can request to cancel it now but a buyer can’t actually cancel a purchase,they can only request to do so. The seller has to agree and initiate the cancellation and since they aren’t replying now they probably won’t reply to the cancelation request either.
If the seller does file unpaid item claims, you should be able to phone ebay, explain why you didn’t pay and get the unpaid strikes removed. It really isn’t fair that you have to go through all of this when you offered to purchase in good faith but I don’t know what else to suggest.