If you own the car and you've lost the title then you can go to the DMV and put in for a lost title and pay a fee to get a duplicate title mailed out to you for the vehicle so you can sell the vehicle with the title.
If you can't get a title for the car and someone just wants to buy the car for parts for another car then as long as you sign a bill of sale then you should be able to sell the car to someone.
If you're selling the car for scrap metal then most scrap metal yards will require a title but you may get lucky and find one or two who will buy the car for scrap without a title.