So it's fine to reply eBay messages using the email reply (not the imbeded respond button)? I mean there are no cookies, or beacons, and they're not seeing your IP so you're ok to reply to messages, right?
NO. Apple's mail app will include your IP address in the message header.
Gmail app does not do this.
Well provided you're on the cellular network and reset your IP between users, the IP they see will be that of your cellular carrier, correct? You just have to make sure your not on your home wireless IP.