Properly terminating a purchase agreement is important. Ask your agent, your broker, and your attorney about the terms of your specific agreement. Usually, as you described, you would have to issue the buyer a Notice to Perform prior to termination. You may be able to accept another buyer subject to termination of the agreement and cancellation of the escrow, again consult with your agent, your broker, and your attorney.
We write our agreements on behalf of our sellers to avoid the problems you describe. It is far too common for buyers, especially investors, to get their offer accepted and then use these time periods to lock up the property while they decide if they really want it.
I am a real estate broker qualified to advise you on California real estate matters, not the law. If you desire legal advice, consult with an attorney.
Best of luck to you!