I've seen a lot of Pawn Stars in the past six months. They never say this, but you can put it together. Most things that people collect trade in low volumes, so pricing is hard, and the bid/ask spread kills you. If you decide to sell, you'll either get 70% of the value if you sell it fast, or maybe 100% if you're ready to wait two years. If you want to liquidate everything, it might depress the market. There's also a discount when you have it as a collection because someone could part it out, but that's even more time. That, and the drive to collect is building the collection, so there's less utility in getting an instant collection.
As a collector of memorabilia for a band that's been around a while: The items I was drawn to, either because I just loved the look or because they were from a show/tour/album I particularly found special, have often turned out to be the ones that have appreciated much more in value than the average thing of that level of rarity for the band.
Also, have you seen The Booksellers?
It took me a while to realize I was collecting books. Before, the way I thought of it was "I would never walk past an out-of-print book about computing history! I might never come across it again!" But now I am a bit more focused on the collection aspect, for example, there are now specific titles and authors that I seek out.