I'm not necessarily disagreeing with you, but it's my experience that stuff produced exclusively to be collectable, that is, "signed and numbered"-type bullshit never increases in value over the long run, and rarely in the short run. This may well be different in the animation arena, but have you ever sold a limited edition "cel" that nominally increased in value? Or is it that sellers who are sitting on them for too long increase the price to make them look more valuable?