I’m not at all knowledgeable about music but I can spot a good sell-through rate when I see one. And certain SEALED blank cassette tapes have a GREAT sell-through rate. Singles and lots are easily going between $25.00-$200.00.
Again, it will be totally worth your time to search “blank cassette tapes” on Ebay in the completed items. And here is an article that gives a little information on the subject as well.
“Fast forward” to you finding some!