Apparently instead of topping up Alipay, I can pay using my linked credit card. For example I have linked my malaysian credit card in alipay (but can't use it to top up balance).
When I check out in Xianyu or any apps after scanning the Alipay QR, instead of paying using balance, the credit card can be selected as the payment method.

So far it has worked for me. Credit card conversion rate is still cheaper than third party agents.