Purchasing used car seats at yard sales etc is illegal! The seat could have been damaged or could easily break because it is used. Goodwill doesnt even sell them because it is against the law. Save a life and buy NEW. The seller was wrong to do this. Never save a dime when it comes to safety! Strollers however are ok to re-sell.
Figuring out those straps are tricky and can cost a life if not done right. If you insist on keeping it, make an appointment with the calif. hwy patrol or police dept in your state and they will install the seat safely and propery for you. Do not do this yourself,please!