title: Authentication system for a web service
body: <p><br><br>
  I am building a web service which i will be launching in near future. Service is more or less like online classifieds.<br><br>
  Now, i need to <strong>build a mechanism to collect user's information, enough to trace him</strong> in case of any fraud with other users. I can ask for nation ID card and things like that. But problem is that how will i verify them and the person providing the information.<br>
  So, i need suggestions for such system which could be used t authenticate users, so other users can trust them. And contact them freely knowing in case of any mishap authorities can get its information from our service.<br>
  For solution we must consider that our service will be free so if this process is costly, than their might be a mechanism to get that cost paid by users.<br>
  Any suggestions will be appreciated.</p>