This is a serious violation of the agents fiduciary duty to you. Ask the would-be buyer or their agent for copies of:
1: The Offer, in it's entirety. Pay attention to page two, as it outlines costs that the buyer requests you to pay. If they were giving you a high offer but then requesting tens of thousands of dollars back for closing costs/repairs/termite work, then your agent may have had good reason to not present the offer.
2: Their financing package. If they were approved by a bona-fide bank lender (Wells, BofA, Etc.) then they were legitimately pre-approved. Conversely if they had a prequal from a lender or broker you have never heard of, perhaps your agent didn't think they were able to obtain financing.
3: Their proof of down payment funds. As Cuba Gooding Jr. Would say "Show me the money". If they didn't show their down payment or didn't have enough, then they probably could not have closed anyway.
If you find that all three of these were present with the would-be buyer's offer, then you have a complaint against your agent. I would first discuss it with them, then their broker. If you don't get what you are looking for try the California Association of Realtors at 213-739-8350. If that doesn't work, go to the Department of Real Estate at http://www.dre.ca.gov.
Let me know how it ends up.