I want to buy something on Ebay US, I choose “pay with credit or debit card” and checkout, then ebay tells me"We can’t process your payment. Please select a different payment method or check your payment details." What could be the reason?
I can choose “pay with paypal” and add monzo account into paypay. But the only thing matters is that under this method I am forced to use ebay exchange rate which I assume not good as monzo rate.
You can change your settings in PayPal to prevent this happening and use the Monzo exchange rate. It may be part of the purchase flow - when it presents the exchange rate offered by PayPal you can select to use the card instead.
eBay may be blocking your transaction based on IP etc. as it’s somewhat more unusual for someone with an IP in China, to be using eBay in the US, with a UK debit card. None of them match so it’s likely just an automated risk decision.
Edit: On the checkout page, it will present you with the conversion, according to PayPal Help pages there will be an option ‘Other Conversion Options’ and it will present the option of paying them without conversion and this will let Monzo handle the conversion.