Messages
Message from the order itself, so the conversation stays attached to it.
Every order, booking and listing has a message button. Using it keeps the conversation attached to the thing it is about, which matters more than it sounds: six weeks later, nobody has to work out which order you were arguing about.
In any conversation you are signed in for, messages appear as you send them and you can see when the other person is typing. A help conversation opened without an account is simpler — you will need to reload the page to see a reply.
If a conversation is going nowhere, either side can ask the marketplace to step in. Both of you will see a line in the conversation saying support has joined, and from that point we can read it and reply. Before that, we cannot read it at all.