I have a small travel agency that sold two airline tickets for passengers made their journey on january 19 2013. On january 4 passengers called the

There are two ways that they might sue you, in Contract, or in Tort. In Tort they would claim that you were negligent, that is that you had a duty to them, that you breached that duty, that theysuffered a harm (damages), and that your breach of that duty was a proximate cause of their harm. You just have to show that you did not make a mistake, or that they suffered no harm, or that their harm was boot caused by you. In Contract, most states have a threshold above which the contract  must be in writing to be enforceable . Check your local State, it could be as little as $500.. Again they would have to show damages, which you say don't exist. They would also have to show that you breached the contract that you had with them. Breach means to not live up to what you were supposed to do. Minor breaches that do not cause any harm are usually not a problem, however each Court will be the judge of that. Bring all of your records to Court. If this is for a substantial amount of money, you should have an attorney because he/she will know how to ferret out a lot of the nonsense through discovery, which is the pre trial process of asking questions about their case and evidence through depositions, interrogatories etc... Be aware that there are very specific rules (evidentiary rules) about what evidence is admissible at trial. There is no way to cover such extensive rules in this answer. A general rule is that yin you don't haves lawyer, make sure to copy theother side on this evidence well before the trial so that they cannot claim that they're prejudiced by seeing it at the last minute. That may also dissuade them from moving forward.