Did the seller do the swap "by the book" or did he try to sneak it past the smog technicians? The law says it's on the seller to provide proof the car passes smog at the time of sale.
https://www.bar.ca.gov/Industry/Engi...uidelines.html
Engine Change Guidelines
Engine changes continue to present problems and challenges to car owners and technicians. Our recommendation is to rebuild and reinstall the original engine, transmission, and emission control configuration.
When rebuilding an engine, it must be rebuilt to the original equipment specifications. However, if you do decide to change the engine, these guidelines must be observed to ensure that the vehicle will be eligible for smog certification or registration.
Remember, these are guidelines for performing engine changes - not certification procedures. All exhaust emission controlled vehicles with engine changes must be inspected by an official Referee station and must have a Bureau of Automotive Repair (BAR) Referee label affixed to the doorpost.
Remember also, state and federal anti-tampering laws generally prohibit any modification to the vehicle's original emission control system configuration as certified by the manufacturer. And, Section 3362.1 of the California Code of Regulations prohibits any engine change that degrades the effectiveness of a vehicle's emission control system.
If there's no BAR inspection sticker, it's not a legal swap. You could roll the dice and make it legit, but IMO it's more trouble than its worth.
Regardless, if he doesn't have a receipt for the engine, walk away. The last thing you want to do is buy it, have it inspected by the BAR... and then discover the motor was stolen.
All in all, this sounds shady.