While commenting is the main form of communication on Listia, there are other ways to interact with other members. You're able to send private messages to individual members that no one else can see. This is where you can get to know other members, talk about other items you are planning to list, or just talk about whatever you want as long as it doesn't violate any terms on Listia.
To send someone a private message, visit their profile page. On the left hand side, you will see the heading "Details", and under this heading you will see a link that says "Contact ______". From here you can send this member a private message.