To understand it better Like advertiser post an ad & he wants to display his contact details in his ad (Most of Classified websites doesn't allow this) here you can post your contact details in the given field. So visitor comes on ads so they can contact advertiser directly. but Most of Classified ask for visitors data for verification and send that information to advertisers and some times they charge for it also. But Below mention websites doesn't work like this it means search for your requirements on this website is safe. Visitors do not have to give his personal details to website owners so they can use or Misuse it.