If you find an ad you're interested in, you can message the owner of the item using the mailbox system.
To send a message to the owner of the item, you first need to be logged in. When you are logged in and viewing an ad the icon will appear. It will be located under the description of the ad
When you click the icon a message box will appear; write your message and press send. The owner of the item will get an email notification letting them know that they have a message.
When the owner of the item responds, you will also recieve a notification to the email address you used when you signed up, so don't forget to check your mail often.