There is no link to a law, because such a law doesn't exist. The state generally does not regulate private party sales of anything, be it guns or oranges or cars.
Specific prohibitions exist against a private party sale where the participants made initial contact at a gunshow (requires that a background check be performed through an FFL).
Specific prohibitions exist against possession, purchase by, or sale to a minor.
Your duty, under federal law, is to not knowingly transfer a firearm to a person who is a resident of a state other than your own, or who is a prohibited person. Colorado adds nothing to this legal burden, other than the specific prohibitions above.