Dude, if you have his phone number, or if you don't, CALL THE FUCKING POLICE, not a private investigator, not reddit, not your friend who works for Apple...

I will tell you right now that the police are probably very dumb when it comes to technical matters. Judges are, too. If you bring a printout of a serial number or an email, that will usually stand as proof in a court of law unless the other party has something conflicting or a "computer expert" who can describe how an email might be faked... Cops, lawyers, and judges are generally pretty ignorant of computers and tend to regard anything printed in black and white as, well, "black and white"-- doing something such as copying an email to text file and changing the "from" text is generally viewed as "hacking", or something similar. If you were to walk in and fill out a stolen goods report with a piece of paper that said "serial number: XXXX-XXXX-XXXX", that would almost certainly count as "proof" in their eyes. They are kind of like mac users in that respect.

It's not that they are all stupid, in fact they are probably vastly smarter than anyone posting here is when it comes to catching bad guys and especially to the laws and procedures relevant thereto, it's just that they are usually accustomed to having other people do the "computer stuff".

In short, you are and have been asking entirely the wrong people. For example, it actually doesn't matter whether your computer has been "sold": it's still stolen goods, and you're still legally entitled to get it back. The seller will know and have contact info for the "buyer".

Moreover, the seller/thief will, 99 times out of 100, be cowed into admitting the theft (or where it came from) in a matter of minutes under police questioning, if the police know it was stolen--the quality of proof rarely matters-- the first step is to figure out who the bad guy is and what he did. Having determined that, the police can usually get a confession, find out where the computer ended up, and can usually get it back to you if it all happens quick enough, no proof and no trial necessary. Cops love it when you bring them this kind of case on a platter.

And "clear" proof that it was yours plus contact info for the thief is all they need. And you've been really vague about how you know it's your computer that was for sale, but assuming it's something like worn keys or some other identifying mark, that's all the cops need. They are not TV police, and they're not going to create a GUI interface in Visual Basic to see whether the culprit is blogging in real time.

OTOH, if you walk into the police station NOW, with a blurry picture of a macbook sold on CL and a claim that you had a macbook stolen with worn keys, that's not nearly as good as if you had filed a stolen goods report the instant it was missing, and included that it had worn keys, and then found one that turned up on CL after the report was filed: police are often quite bad with computers, but very good at sequence-of-evidence and chain-of-custody matters.

Which is why, the lesson right now, yesterday, and tomorrow is always ** CALL THE FUCKING COPS** when you are the victim of a crime. If you have not already called them then you are stupid and deserve to have lost your macbook.