Am I responsible for the smog on a private party used car sell? I let them know up front it needs a tune up.. and that smog wasn't due until October. Aren't private party used car sells.. as is? Im in California.

If you tell the people you sell it to it has the problem and it is in writing in their paper work that it has the problem you should be okay. If the car is over 10 year and If you sell the car for under $1,000. Or well under the Kelly Blue book as is value New Car Prices | Used Car Values - Official Kelley Blue Book Site
is even more to your advantage proving it is as is because most cars under a thousand ARE as is. You can check the
Also they can't come back months later after they purchased the vehicles claiming damage was pre existing to the purchase.
Often people try to say "Lemon Law" when somebody says they bought a bad car but the Lemon Law has exemption rules which generally include
As is
If the car is over 10 year
If you sell the car for under $1,000.
private owner
if they wait (I think over a month or more) to make a complaint.

If you did not disclose the problem (or it is a he said/she said thing) then you may have problems but any of the above things should be on your side