Use email to validate post, not unlike craigslist
You can use the Rules module, but you'll have to write a custom rule to grab the email from the submitted node and send a verification link to the email address. You could also do this with a custom function through hook_entity_insert().

For the verification link you could do use drupal_hmac_base64() to create a url-safe hash of the content of the post or of the node + timestamp + email or something like that. 

Then you can create a custom menu to fire a callback function that takes arguments from the verification link to publish the node if the verification link is correct.