This probably isn't what you want to hear, but there's really nothing you can do.
Reporting your serial number to Apple doesn't help
You can call Apple to confirm that yourself. It's probably because they can't filter out true claims from fake claims. Imagine the hassle the original buyer has to go through because some saboteur reported that buyer's serial number is from a stolen Mac.
I see a lot of people posting screenshots of their Mac specifications online without masking out the serial number.
Registering your Mac with Apple doesn't help
Everyone can reinstall the Mac OS and register their name with Apple. Everyone. So what's preventing someone from registering the stolen Mac? Nothing.
Apple will however continue to honour the warranty coverage even if the Mac is stolen, provided it is still within the coverage period. That's basically saying that the secondhand value of stolen Mac is no different from that of a non-stolen Mac.
Once your Mac reach the secondhand market, it's a goner
There are tracking software that can be installed to take photos of the thief when they are using the stolen Mac. The question is, how do you know it is the thief that's using your Mac and not a genuine secondhand Mac buyer?
I've people arguing with me that they will snatch back their Mac from the genuine secondhand Mac buyer. My question to them is, whose mistake was it that allow the transaction to happen in the first place? Answer is, of course, the original Mac owner. I had to write this one out explicitly because it might be hard for grief stricken owners to comprehend.
One cannot expect the secondhand buyer to call back to Apple to check if the Mac was stolen. If you've read earlier, you'll know that Apple doesn't care about whether your Mac is stolen or not.
I've bought a lot of secondhand Mac before. Never have I made any effort to check with Apple to see if the Mac was a stolen property. It just doesn't matter even if it was stolen. There's no difference for me, personally.
Even if I know the Mac was stolen, I've no way to prove that the seller didn't get his/her Mac from the secondhand market.
Move on
Yeah, it's sad when your Mac is gone. Face the truth. If it's lost, maybe some considerate person might return to you, provided you have your address or contact information on your Mac. If it's stolen, it's pretty much gone for good.
I've never read of any person who successfully recovered their Mac other than those who write on those Mac-tracking-applications websites.