1) Is it worth it to sue for $3,000--it depends on the value of  your time. It's probably not worth retaining an attorney, but it would be worth bringing a small claims action, acting as  your own attorney, so long as you would not make more in, say a day, then you hope to recover from the lawsuit.2) If the seller misrepresented (lied about) the ability the car to pass a smog check, that would be fraud, and should provide grounds to rescind the contract (return car, get money). It may also be a breach of contract in that you paid for a car which is  road legal in all ways, but were not given that. Therefore, you should have grounds to sue.3) You cannot sue someone without an address, since without an address, you cannot serve the court papers  (summons and complaint) on them; without good service, which meets the requirements of court rules, you cannot establish personal jurisdiction, which means the court would have no legal authority over them.However, you may be able to sue without a name, if you have the person's address (though best would be if you find out his name, using address or phone number and any of the many search or look-up tools on the Internet)--you can file the lawsuit initially against a "John Doe," describing him by his actions (e.g. "the seller of a 1998 white Ford Mustang, VIN # . . .. ") and by the other identifiers you have (e.g. phone number, address); then later, when you get the seller's name, you can amend the complaint with it.