I find myself looking for a general principle of what to keep and what to pitch, and I'm not sure there is one.
Some things are straightforward:
- I'm getting rid of most of my D&D books, except for the three basic books. I don't expect to ever play D&D again; it's not what I want in an RPG now. I'm happy to keep a few books as souvenirs of the campaign I ran in the early 2000s. But I have a lot of splatbooks that are just collections of feats, and I mostly didn't care about those even at the time. (I wonder who those were written for; they weren't very helpful to me when I was playing.)
- I'm keeping Robin's Laws of Good Gamemastering, because that's something I expect to be able to use in any game I play.
Other things are more uncertain. By and large these are things I played once (or wanted to play once upon a time), but don't expect to play in the future. My circumstances have changed; my roleplaying will probably be occasional at best, so for actual play I look for RPGs that are so streamlined that it's possible to play a complete story in a few hours.
Some games for which I have collections and no expectation of playing:
- Exalted. I loved the "mythic heroes" aspect of the game, but I never played. I even bought blank D10s with a plan of making dice rolling easier. I have several books.
- Over the Edge. I liked the surreal setting and the flexibility of the system, but I've never been improvisational enough to play that well. I have several books.
- Unknown Armies. Another fascinating setting of the occult underground, another game I would have trouble running or finding players for. I have a handful of books.
- Mage / Vampire / World of Darkness. I never actually played these when they were cool. But when I pick them up, I remember the friends who recommended them and why.
- Feng Shui. Fun action movie setting, system has good points and crippling flaws. But I actually ran one adventure with this and we had fun. I have several books.
- Buffy. I liked the way it tried to balance Heroes and White Hats, and I liked the show. I've got a few books, and I'll probably keep them.
- Castle Falkenstein. I liked the magical-steampunk setting, and I've even played in a Falkenstein LARP or two. I've got six books, which means a complete collection.
- Toon. I've had a few good games of Toon in decades past. It's a simple system, but not actually well-suited to modeling the cartoons that it claims to model. It works better as a less-complicated GURPS. I'll probably keep these.
- GURPS. I did a whole lot of demoing games for Steve Jackson Games in the late 90s, and as such I have a nearly complete set of sourcebooks for GURPS 3rd Edition. But GURPS 4th Edition is ten years old. (GURPS 4th is the system my friends are most likely to play, and I'm likely to keep my few GURPS 4th books.) The sourcebooks would be pretty easy to adapt to GURPS 4th; the rules didn't change that much, and the books are much more focused on history and scholarly treatments world detail than, say, D&D splatbooks. But the books tend to be dry as toast. Purging GURPS books is probably my biggest opportunity for regaining shelf space, but I was happy with my collection once and feel a twinge of angst when I think about getting rid of them.
(If you're interested in any of these, let me know. I'd be more inclined to part with a collection that I knew someone else wanted.)