Actually no state has a "buyers remorse" law that I have ever heard of from any of the attorneys that I know across the country. Also, in a private sale between two people, generally the only obligation on the seller is to answer the buyer's questions honestly and not hide anything that they should know that the buyer would want to know about before buying the vehicle. That and turn over a clear title to the buyer. In those states that require vehicles to also pass an emission inspection, it would have to be able to do that too. But the law can be a little different in each state and you should talk to a local Consumer Law attorney to find out for sure what your state law says. All of that is the "legal" answer. The "moral" answer is that perhaps you should also consider the situation, what was said by you during the sale, the buyer's mechanical sophistication and your own, and what you think would be the "right" thing to do. Generally speaking the law tries to encourage people to do the right and fair thing and as long as you take that approach you will usually not run afoul of most legal obligations. So, do you have to fix the truck? Probably not. Should you help get it fixed? That is a question that you will have to answer for yourself.
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I've seen this scenario repeated again and again over the years and almost always from the other side of the fence. Assess your knowledge of the vehicle, buyer's knowledge of it, what was said and then put yourself in the shoes of the buyer and I'm sure you will be able to work something out...
It is more than a little disappointing that the out of state attorneys are willing to "answer" your very specific question without any knowledge of Idaho law, and with such a strong bent toward morality.
Our state statutes prescribe a rescission period of 3 days when a sale is solicited at the home of the buyer. Case law has not extended this right to private casual sales of the type you describe. If you made it clear to your buyer that the sale was "as-is/where-is" and he paid and took possession on those terms, you owe your buyer nothing further. You don't disclose what caused the break-down, and you will never know what happened in the intervening 20 minutes, so engaging in "negotiations" is all downside risk for you.
Best wishes for a favorable outcome, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.