Steve:
The answer to your question depends upon what state (jurisdiction) you are in, but some general principles of tort law would likely apply in most jurisdictions. Since you are not in the business, per se, the strict liability tort law concepts that are used to tag GM or Ford should not apply. You would likely be held to an ordinary negligence standard, ie: reasonable conduct under the circumstances.
The safest thing to do when selling a homebuilt car is to prepare a short sales agreement setting forth the price and terms of payment, and also stating that you are not giving any guarantees or warrantys whatsoever, that the buyer has inspected the car, is satisifed with its condition, and (here is the important part) that the purchaser releases and holds you harmless for any defects, flaws or errors in the components or assembly of the car. I have worded this in more layman's terms rather than legal terms, but I suspect you get the idea. Both you and the buyer should sign it.
There are few absolutes in the legal business, but a short sales agreement where it is clearly stated that the buyer has inspected the car and is taking it 'as is' should go a long way to protecting you.
Keep in mind that imposing liability upon a non commercial casual home car builder generally requires that you 'knew or reasonably should have known' of a faulty part or error in the construction of the car. Obviously you would not knowingly do something negligent, so it would be very hard to impose liability upon you in such circumstances.
I am not aware of any reported cases where a kit builder was held liable for a defect discovered after sale of the car to a third party, although I have not specifically researched the issue.
The statute of limitations period for tort cases varies from state to state, but is usually two or three years, usually measured from the date of injury. Many states have what is called a 'statue of repose' for manufactured products, often ten years from the date of manufacture, but it is questionable whether that rule would apply to a home built car.
It is possible that your homeowners policy would provide you with liability protection in the event you were sued for negligence in assembling a homebuilt car, since the coverage afforded is usually broad. But policies vary, so it may depend on the 'fine print.'
Hope this helps.
Chuck
