I'm going to buy something for the sake of being a good customer -- I can always get some modelling materials even if they don't have anything I specifically want -- but really what I like the most is the fun of bargain hunting. You never know what you're going to find! I've actually been much more successful in the miniatures field than in RPGS; my collection of Rogue Trader era rarities doesn't mean I was picking up weird Adeptus Mechanicus models when I was 9, it just means I am assiduous in my searching of flea markets, car boot sales and of course the internet.
But that's not to say I haven't found a few cool things second-hand in the RPG world, mainly thanks to my old games shop having a good used shelf but also thanks to the various online communities I'm part of.
Now, as you might expect, most of the games I talk about in my second-hand post are out of print, but not all of them.
You can't find this specific Glorantha book, but Design Mechanism has some other ones, including many for $1. Those are only a fraction of the many books out there, I'm just saying that they're from the same era as the one I mention in this post.
And of course Reign is still available. And if you just want the leading-large-groups rules and not the setting, you can get the Enchiridion.