Craigslist does that with phone verification. You have to take a call or SMS with a code to enter on the website before you can post in a high-spam-risk category. That ties accounts to phone numbers which creates a much higher barrier to re-registration than just e-mail addresses or Facebook profiles, and makes the account traceable if authorities need to get involved.

That's part of the reason I created http://www.dialshield.com, a widget/API that lets anyone add phone verification to their website the same way Craigslist does.

The site doesn't look like it's limited to trading within social networks, so why limit logins in that way? If you expect this to stamp out abuse, it's not going to. People can just register for a new facebook/twitter account and then use that...

I'm with the above poster, I generally avoid Facebook logins.