It makes sense; people interested in drugs are presumably also interested in other things, and ordering drugs online somewhat implies a certain level of expendable income, so it'd be a decent marketplace for not-drugs. And I imagine drug-hobbyists probably have more diverse interests than the average person, as they've already expressed interests in the not-publicly safe hobbies (but likely still also have safe hobbies as well)
Knock-offs, and stolen merchandise are often sold by the same criminals who sell drugs. Aside from street dealers who are dealing to get their next fix, it’s not as though drug dealers have some need to restrict themselves to one lucrative market.