If I sell a used vehicle as is, can they come back the next day insisting on their money back, they say there is a lemon law, but the vehicle is still just fine, passed emissions, please enlighten me on what is legal.

Lemon law is only for new cars

Don't quote me on any of this. I'm not an attorney, but from my understanding the lemon law wasn't created to cover the buying of a used car from a private party. The lemon law was created to keep car manufacturers/dealers from ripping people off. Basically it helps buyers of new/certified used cars in cases where multiple things go wrong with their new vehicle within a short period of time. If that happens the dealer falls under obligation to fix the problems no cost, exchange the vehicle for one of the same, or refund the cost. Again, the lemon law wasn't for covering a party who knowingly bought a used vehicle with problems. A private party only has so much ability to check their vehicles and as long as the seller was honest with the buyer then it falls back on the saying "buyer beware". If there was found to be some deception then the buyer might have a case.