I used to stock a serious amount of filters. Cokin, Tiffen, Schneider B+W, and Satter econo-line. I would guess that in the hey-day I probably stocked 200+ different filters of all styles and sizes.
But the most fun was when a customer would request the filter that most matched their sunglasses. Of course they would hand them to me to look through to check for the proper match. I even had a customer ask me to match a filter to the dark area at the top of his cars windshield! He liked how the sunset looked through it.