If you possess older, out of print role-playing games, never get rid of them, ever. If you have a chance to pick up older, out of print role-playing games at a bargain, do so immediately.
One thing that's really astonished me about RPGs are that they age the most gracefully out of nearly any form of entertainment I've ever seen. The crustiest, crudest print of an old Dungeons & Dragons book still fetches a handsome price on the market. More to my point, though, are that those books are still very playable, and oftentimes just as fun as they were when they were first purchased.
D&D is a good example of what I'm talking about. Being one of the oldest RPGs out there, there is a lot of older material for the game. One would think that stuff would be old news, obsolete, and no one would be into it. One would be very, very wrong. There is literally an entire sub-community within the greater RPG community completely devoted not only to old D&D stuff, but to the printing and playing of brand-new material purposely designed to look like the old stuff!
There are a few reasons for this particular trait amongst RPGs. One is, of course, the nostalgia factor. Just like action figures and 80's cartoons, there are quite a few collectors of older games out there who just like to hang onto these artifacts of their own adolescence. Another, more practical reason is that many RPGs didn't have the financial support to stay in print. There was nothing wrong with the game; there just wasn't enough money to keep it alive. There are some true gems out there that would be huge in today's community if they were still easily accessible, games like Alternity and Everway and the old Dragonlance RPG powered by the card-based SAGA system. I owned all three of those games, and I now kick myself daily for ever getting rid of them.
Even older editions of still-existing games still carry a lot of value. Take the World of Darkness, for example. When that universe famously rebooted itself, it of course left behind a lot of die-hard fans of the original settings. For them, the old books are all they need, and that system hasn't actually changed all that much since it first appeared, over 20 years ago. To my knowledge, I can think of very few newer editions that rendered older editions completely obsolete. GURPS comes to mind, but that only applies to its core books: there are older edition sourcebooks that have yet (if they'll ever) see a current edition reprint.
So, because people like lists and it pads my own word count, here is a top five list of completely out-of-print RPGs I would totally pull the trigger on if I had the chance to today:
1. Aberrant. The World of Darkness spin on superheroes, this was one of my all-time favorite superhero settings. It is currently available in pdf form, but I'm afraid the scan would be sub-par quality. A print book would have to do it (and, as memory serves, the printed book had the dimensions, shape, and feel of a graphic novel, which made it all the more awesome).
2. Wraith: The Oblivion. Another World of Darkness game, Wraith never got the "New World of Darkness" treatment, instead kindof evolving/devolving into Geist. What made Wraith incredible was its world, a ghostly spirit world where weapons and buildings were literally made out of the souls of other people. It was a game of horror and redemption, and young, teenager Ed loved every page of it. It's getting the "20 year anniversary" treatment from White Wolf publishing in the coming months. It'll probably be the one re-issue I will actually buy.
3. Earthdawn. What I liked about it was that it was the first RPG (if not THE first, the first I ever saw) who's fantasy world was built for role-playing, rather than for literature. There was a reason dungeons existed. There was a reason why your character was more important and powerful than normal people. Levels weren't just abstract number comparisons; they actually had a magical equivalent in the game world. It was really awesome. When it was announced that a new edition was being funded via Kickstarter, it quickly became the first game I ever backed.
4. Alternity. Basically D&D in space, Alternity consisted of three gorgeous hardcover books. With universal mechanics and a framework that could allow for any sci-fi campaign setting, it was to be my go-to game whenever laser pistols or aliens were involved. Sadly, it came out at a time when I wasn't playing many RPGs, and I never got a chance to play it before I sold it all on eBay.
5. The SAGA System. There were two SAGA system games: a Dragonlance game, and a Marvel superheroes game. I owned but never played both. They were both card-based; you had a hand of cards that worked as your dice rolls and your health. Since you had to play cards to succeed at tasks, many games took on a more tactical feel; do you burn your highest cards to assure success? Or do you save them for when you really need them? Between the two, Dragonlance was the cooler one, as it was more widely supported, but the Marvel game was really impressive, too.