Same here. I get the feeling this was a pretty common hobby. I would also pre-format them and charge a little extra. If you could afford to buy a batch of a thousand or so unformatted single sided floppies, doubling their capacity and formatting them, you could make some pocket change.
Someone else started this first but the floppies they were selling were crap so...
This was also a period when I earned money working in the school's computer lab to help students and there was more than one occasion where a student had their semester of work on a floppy and they bent it or otherwise made it difficult to read though I could usually get most of their info off.