It depends. Selling is an automatic disqualifier (as you showed you are willing to commit a crime for money). Past usage is not an automatic disqualifier, and

It depends. Selling is an automatic disqualifier (as you showed you are willing to commit a crime for money). Past usage is not an automatic disqualifier, and shouldn't be a problem if it was years ago. Lying is an automatic disqualifier, if you get caught (and perjury if they decide to pursue). However, it is of my opinion that you should only selectively share information, and only the information that they are likely to learn in background checks. I had a security clearance about a decade ago, and know many others who received them and other who were denied. The ones who were completely honest were denied. However, this was also immediately after college, so there wasn't ten years of clean living in between. (This circle of friends also experimented with pretty much every substance available during college.)