From dropdown you are selecting general location of your advert while selecting it on the map you are being more precise. Also if the location search of the marketplace is set to geolocation, people who search by location will find your ad more easier. Location sSearch of the marketplace can also be set as list of predefined values in which case value from the dropdown is being used.
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 have found some ad which you want to visit later or contact seller later, etc. you can mark it as Favorite and it will be save to the list of your favorite ads which you can access from your profile dashboard.
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.