I was trying to pay for something that was in US, to be shipped to a US mailbox and forwarded to Australia to save on freight.

Every attempt to pay with my credit card or Google Pay failed with the very unhelpful error message:

Please check your payment details or use a different payment method

I checked my payment details, checked with my bank, even checked with Visa - who saw a transaction was processed and then reversed a minute later! Of course, eBay was zero help at all.

I realised the seller only sold to US buyers - this means they don't have to pay an International Fee. Even though I was sending it to a US address my primary address was still in Australia. My guess is that eBay rejected the purchase because an International Buyer was trying to purchase from a US-only Seller, and eBay couldn't take there large cut of the transaction.

So how did I get it to work?

Changed my Personal Information address to my US shipping address, paid, and then changed my address back. It means I don't get some of the eBay protections but I trust the seller.

I hope this helps others!