You're fighting an uphill battle. When private parties sell a used car, the sale is ALWAYS as-is, unless there is some specific agreement to the contrary.
before purchase you could have asked for the current copy of smog check paperwork and make a discision from there or ask if you could take it down for testing.
if a person say it is in good condition it don't mean it will pass a smog test. you could have taken it to your mechanic first. also look it up on the net.
private sales between individuals in ca(Avvo) site may answer the question

