This is too involved. You need to consult personally with a lawyer, the internet isn't going to solve your problem. Good luck.
I hope you found my answer helpful. If you did, please click "helpful" and/or "best answer," because doing so helps my Avvo rating. Please also understand that answering this question doesn't make me your lawyer, and my answer is not even an "initial consultation" with me. In answering your question(s), I am merely giving you VERY GENERAL ADVICE based upon your VERY GENERAL QUESTION. If I indicated that you need to see a lawyer personally, then PLEASE DO SO. Good luck to you!
Mr. Castagliuolo is correct, you will need to contact a consumer attorney and have him or her review all of the paperwork you executed at the time of sale and research whether or not the title to your trade-in has been branded. While ordinarily there is no legal requirement for a consumer to disclose the history of a trade-in vehicle I have reviewed a number of vehicle sales files that contain language that require the purchaser to convey "marketable title" or have other similar requirements regarding trades. Each case is different so a detailed review of the facts and the documents is necessary.
Disclaimer: This response is based solely upon the limited information provided in your question; additional facts may be necessary and may lead to a different response. The attorney responding to this question is licensed only in Florida and this response is intended for informational purpose only and not intended as legal advice in your particular state. Additionally, the response is not intended to create an attorney client relationship.
You state, " I get a letter from a lawyer saying they were suing me. . . ". That pretty much answers your question. Yes, they can sue you. Now get a lawyer. Given the facts as you state them, Geico looks to be a wonderful third-party defendant, and you a third-party Plaintiff. Ask your attorney what I might mean by those statements.
While I'm Board Certified in Construction Law with years of contracts and litigation experience, and I still warn you that with any free advice you generally get what you pay for.