What actions can i take against someone who sold me an unregistered car that needs a smog check?

You'll need to look up the laws in your state. In Ca it is the responsibility of the seller to make sure the car can p smog. If it does not, they are required to either spend what the vehicle was purchased for to fix it or refund your money. Sadly, you will have to go to civil court to resolve this matter. Many times though, being served will be enough to get the seller to do the right thing. Again, you should post what state you are in and you can find the laws very easily online for your state. Best of luck