I can't say I'm any kind of expert in the art of stamp collecting. Though in my limited insight I know stamps are usually most valuable when untouched. Though postmarking is so much a second nature with the sticky squares that it doesn't hurt the value that much at all if a used stamp has a post cancellation inked over it. Actually certain rare postmarks can increase the value of a given stamp and some just collect postmarks more than what their stamped onto.
Of course, cover fading isn't preferred and isn't going to increase the worth of any tape, but this phenomena shares similarities with postmarking. Both are so intrinsic to the hobby that it's just automatically accepted for the most part. Sellers often don't even mention this and buyers usually don't blow a gasket. Some tapes are so rare that it's not worth bitching over such damage. Most of these tapes are passed along from collector-to-collector and otherwise kept in great shape. So if you're new to the world of Japanese cult pre-records, just remember these thoughts to save you headaches over the sun's rays on your purchases. It's as common as how uncommon (and plain cool) these particular tapes are.
Just a little notion today since I was too beat to watch a flick last night and nothing arrived in the mailbox today. I did just complete one of Craven's '80s minors so expect my head meat meanderings tomorrow...I want to hear you scream...