SEE UPDATE BELOW
I was so intrigued to find someone paying over $100 for an old blank cassette tape that I've been looking into this shady network...
First up, I found German-based el355 who recieved 12 bids of up to $474.99 for his collection of 70 sealed tapes (including a Cobalt 52 and various Teac Open Reel gear). He was looking for $1,200, so he didn't sell.
His bid list is a who's who of the tape-collecting world, featuring big players like Jessydog (a rare US collector), who recently dropped $25 on a pair of BASFs, and efeler who spent almost $45 on a pair of Nakamichis.
The top bidder for el355's collection was another German, Heimoboris, who last month paid $112.49 for three Teac metal-reel cassettes. (That's one on the right, with the original $12.45 price sticker.)
Another American dealer is ceogc02 who also trades (inevitably) in WWII memorabilia.
I've been very disappointed not to find "Vintage Cassette Collector" magazine on the web, but this post (which does feature several Germans) gives a taste of what it might be like.
UPDATE So, most of the ebay links on this page have decayed, but a look at 'Blank Tapes' category in eBay shows that collecting is still going on: As I write, there's a "Sealed! Mint!" That's tape on Buy It Now for $30. Ten Sony Metal Masters recently went for $225.