You should go and get legal advise immediately. Have the attorney look at the contract and see what is really going on. Is the broker/reattor represening both you and the real sellers? What does the purchase contract say? Which stage of the purchase are you in?
I have so many questions to your question above, we will need to know a lot more details to give you advies. Why are you give HIM the downpayment instead of going through an escrow company (which is normally the title company in CA)? Did he sign the contract or the other person? the list of questions go on and on.
The problem is there are so many scams out there now because a lot of people are trying to get what they perceived as a great deal and ended up dealing with very questionable parties. Buyers and sellers should be extra careful now a days and make sure they are dealing with legitimate parties who knows what they are doing
Did the realtor really tell you he's the owner? Then yes, he misrepresented himself.
Writing a cashier's check payable to the seller is a big no-no. It should be made payable to a third and netural party like the title/escrow company who should hold that check in an escrow account that neither the buyer or the seller can access without mutual consent.
Did you execute a purchase agreement? In California, purchasing real estate property must be in writing (contract).
Did you open escrow at the same time you wrote the check? Did the escrow officer give you a preliminary report? Does it specify who the owner is?
Do you have any contingency periods ---- appraisal, loan approval, inspections? If there is a contingency period and you back out before you clear the contingencies, and you cancel the agreement, you have the right to ask for your deposit back.
AND....check out this realtor/broker's license status on the California Dept of Real Estate website. http://www2.dre.ca.gov/PublicASP/pplinfo.asp
You have every reason to want to get out of this agreement and get your money back, even if you want to buy the house.