If you have performed a search accurately, and delivered it to a client on time you have performed your end of the agreement. In the absence of an express agreement you can sue on a number of other theories of recovery.

You can sue a client on a contract implied in fact. This is a type of contract in which no spoken negotiation has occured between the parties. The court will examine the conduct of the parties to determine their intent. The best example of a contract implied in fact is your conduct at the super market. When you take the groceries out of the basket and place them on the counter, your conduct is an implied offer to buy them. The clerk's conduct in ringing up the groceries and bagging them is an implied acceptance of the offer. Your payment for the groceries, and the clerk's release of them to you is the consideration for the implied contract...perfectly binding on the parties.

You can also sue a defendant on a contract implied at law (aka quantum meruit, unjust enrichment or quasi contract). If a defendant accepted service from you when he knew or should have known that you expected to be paid, he is liable to you for the reasonable value of your services. Landscapers use this argument frequently when they plant shrubs at the wrong property. However, the court will not enforce mistakes and carelessness. I succesfully defended property owners who were sued by an oil company that delivered oil to the wrong address. My argument in their defense was the Plaintiff's carelessness in making the delivery..

You can also sue on express oral and written contracts if the facts support such a claim. It sounds as though you have and open price term with respect to the time of payment. In this case payment is due in a reasonable period of time. To determine what length of time is reasonable the court may look at industry standards or a prior course of dealing between you and your client.

You can also sue a defendant on a tort theory of conversion (civil theft). The search remains your property until it is bought, and payment is made. Although tort claims are generally harder to prove than contract claims... particularly with the issue of a defendant's intent. In Connecticut we also have an unfair trade practices act which is easier to prove than tort cases. It also provides for punitive damages and recovery of attorneys fees. I have serveral cases pending now in the court. Most states have similar statutes. California probably does too.

You might even look into filing a complaint against a problem client with the appropriate police dept for theft of services. However, a lot will depend on the language of the statute and how lazy the cop is that takes the complaint. Very often here in Connecticut the police do not want to involve themselves in it, and will tell you that it really is a civil matter rather than criminal. Maybe you will luck out, and get a cop that is willing to get off the dime. The court will generally make restitution to you a condition of extending leniency to a defendant.

There are numerous remedies available to you. Do not let anyone push you around. At the very least you will get an opportunity to vent your spleen. You will feel better afterward. I really have some difficulty understanding the number of postings I see here concerning non-payment, and the hesitation of those posting to pursue their remedies.