I have contacted eBay support today, they said there is a technical issue at the moment and they're actively working on that.

Hello Andrew, please note that Afterpay is having some technical issue because of which the same is not going through. Similar like you we've had other customers as well since today morning who have been facing the same issue. I would request you to wait till tomorrow and wait to make the transaction as the same will be resolved by then. I can understand that the AfterPay payment option is not showing up on the checkout page. Please be assured, I am here to help you with best of my efforts. We regret the inconvenience it has caused to you. I have quickly contacted our dedicated technical team and came to know that this is a global issue where the valued buyers are not shown the AfterPay payment option. Actually, we do not have a dedicated payment gateway of our own, all our payments are processed via PayPal checkout. Here, the PayPal dedicated payment gateway is experiencing AfterPay outage due to which the option is not showing up. Please allow me to share that our technical team is working on it to resolve it as soon as possible. We have got an update that this will be resolved within 36 hours. Please accept our sincerest apologies for this. Sometimes, even the best computers face issues and bugs like this which cause inconvenience to our valued members. It is in times like these that we badly need your support as we do get really anxious that our customers are not having a good experience as we always aim for.