You respond to an ad by using the email function built into this site.  When ads are posted there is no need to display the email address of the person that posted it (to reduce spam).

When you are viewing an ad which interests you, look at the bottom of the ad.  You will see two choices; "respond to ad" and "save ad as bookmark".  Click "respond to ad" and follow the prompts.

Your user name and email address will automatically appear in two boxes.  This will be shown in the email which is sent so that the email recipient can respond to you.  Should you wish to provide a telephone number or something, just add it in the 'comments' box.  

Also in the 'comments' box, you may wish to provide additional information in the email.  Perhaps you wish to ask about shipping or payment method, or to suggest that 7pm is a time to call.  It is usually helpful to give an idea where in the world you are so that they can give you an idea as to shipping costs.  You don't have to enter anything here if you don't want to.