You would have to examine if it is a controlled/scheduled substance. Other rules may apply too. For example, spray paint is not illegal, but selling it to a minor in your state may be, same thing goes with the anti-meth-ingredient ID laws some states have for ephedrines. I think there are no laws prohibiting you from giving aspirin to someone, but the prilosec may have a 'non-prescription' dose that the prescription exceeds.
There are so many laws
now that it's no longer possible to guarantee that any action you take doesn't violate one of them
That's why it's best to use services like Silk Road, because even if you think what you're doing is legal you can never be entirely sure.
when in doubt, cover your ass. nobody else will and the rest will flip on you in a second if they can gain from it. Most people are basically decent, but need to have a plan to protect you and yours from those who are not. 12 gauge and TOR has solved most of my simple problems, best of luck on yours.