What is the jurisdiction? Where do I file, Colorado or California?

I am not sure I quite understand the facts here. Let me see if I can clarify. 1. You live in Colorado 2. Your purchased camera equipment and used your credit card. 3. However, you were purchasing the equipment for someone else (let's just call them "friend") 4. The camera equipment was sent to friend in California. 5. You have some sort of agreement (do you have it in writing?) with friend that the friend would pay you back? That is a somewhat complex analysis. The default rule is that you generally sue someone where they reside. The issue is having the court be able to exercise jurisdiction (power) over the defendant. Your underlying claim (appears to be) a breach of contract claim. You had an agreement with Friend whereby you would purchase and pay for camera equipment for friend and friend would pay you back. The jurisdiction question is, where is that contract considered to be executed, no way to tell based on the facts you presented. It is safer to sue in CA, or more certain. If you sue in CO, you run a substantial risk that a CO court won't have jurisdiction over hte defendant. As a practical matter, it would probably be more beneficial for you to sue in CA. Reason being, if you sued in CO and got a judgment, you still need to take that judgment to a CA court to have it recognized (domestic the judgment in CA) before you can do anything (seize the equipment, garnish wages, levy bank accounts, etc). You are going to end up in CA court anyway, so you might as well start there. You can definitely sue in CA, the more suspect position is whether you can validly sue in CO.