Section 24007 (b)(2) of the Vehicle Code states it is the responsibility of the seller to provide a valid smog certificate when selling his/her vehicle. There is no provision in the law to sell a vehicle "as is" in terms of the smog check.
Now if the buyer will sign a waiver, admitting full on, that he/she is aware that the vehicle is being bought without a smog certificate because of engine problems or problems unknown to you, you might be fine with the sale. Remember however, if the buyer one day decides that they made a bad choice, they still can sue you to collect damages. Who a judge will side with will be up to the court.