In order to retrieve phone number of the seller go to it’s profile page or any of his ads and you will notice block with phone number.
If you wish to start messaging conversation with the seller first create an account. Once you have created your account navigate to any of his ads and you will notice button for sending message. List of you messages is available on your profile. There you have AJAX messaging system where you will be able to continue conversations.
If you feel that the ad is breaking some rules, you can report it directly from ad page by clicking on the Report button and writing detail reason. Ad will remain active until administrator checks it out.