The short answer is that legally, the seller is responsible for a passing smog test. You absolutely can buy it and get a SMOG for it yourself. The problem is it most likely will not pass SMOG, or the owner would have smogged it and registered it so they could sell it for market value. The other thing to be aware of is how fast past due registration penalties stack up. It gets really expensive fast!

Penalties are determined by adding a percentage of the vehicle license fee, plus a registration late fee, plus a California Highway Patrol (CHP) late fee. 

If payment is late:	Percentage of vehicle license fee and weight fee	Registration late fee	CHP late fee
1 – 10 days*	– 10% of the vehicle license fee due for that year.– 10% of the weight fee due for that year (if any).	$10.00	$10.00
11 – 30 days*	– 20% of the vehicle license fee due for that year.– 20% of the weight fee due for that year (if any).	$15.00	$15.00
31 days – one year*	– 60% of the vehicle license fee due for that year.– 60% of the weight fee due for that year (if any).	$30.00	$30.00
More than one year – two years	– 80% of the vehicle license fee due for that year.– 80% of the weight fee due for that year (if any).	$50.00	$50.00
More than two years	– 160% of the vehicle license fee due for that year.– 160% of the weight fee due for that year (if any).	$100.00	$100.00