C.B.
The DMV says the seller is required to provide the buyer with a current (within the last 90 days) smog certificate. I sold a car once without one but I made the buyer sign a note saying they would assume all responsibility for getting the smog certificate.
If you have an AAA/ CSAA membership (towing service, etc) they have a DMV department who might be able to give you some advice.
Also, make sure you submit a Release of Liability with the DMV. The last thing you need is someone trying to sue you because the buyer never registered the car into their own name.