You have to register an open-air account first. It's simple. It's no trouble at all.
Find the book you want to buy, leave your question in the "Q&A" link in the product, and the seller will answer it for you.
Payment method, you can ask the seller whether to accept remittance from the mainland.
There is no problem with the delivery method. It can be sent by SF Express or Chunghwa Post from Taiwan Province Province, as long as the freight is negotiated with the seller.
I have an open-air auction shop, so I know. For your reference.