If you are the seller, up front say, "don't tell me what you are using these for and there won't be any problems". It isn't that hard.
That would sound very suspicious and may even be taken as admitting that you're willing to help criminals as long as you don't assume any risk yourself.
A better tactic is just to avoid dealing with anyone giving you reason to believe they are involved in criminal activities or a cop trying to entrap you.