Based on the limited information that you have provided, all of the check copies should be sufficient to prove your case. If you can get the individual to actually admit, in writing, that he owes you this money, then even better.
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The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here. Please visit my web site: www.avanesianlaw.com for more information about my services.
You Can Try To Get Him To Sign A Payment Agreement Which Outlines The Amount Owed And Payment Arrangements. If He Signs It And Defaults On It, Which He Will, You Can Sure On The Payment Agreement. Otherwise, You Have A Claim For Breach Of An Oral Contract And The Bad Checks Will Serve As Proof. You May Also Want To Research If There Are Additional Statutory Penalties In CA For NSF Checks. Good Luck Collecting. I Think You're Going To Need It.
Disclaimer: The foregoing answer does not create an attorney-client relationship with Attorney Cannella or her firm. This answer does not constitute legal advice, is provided for informational and educational purposes only for persons interested in the subject matter, is not legal opinion, nor confidential in nature. Each situation is fact specific and may be subject to state specific laws. Without a comprehensive consultation and review of all the facts and documents at issue it is impossible to evaluate a legal problem fully.