I buy a lot of RPG books, with "Does this look interesting?" as my main criterion.
I have a big 4e collection, but it's not complete - the Dragonomicons don't grab me, nor the Eberron stuff, nor most of the adventures. I have a complete collection of 1st ed AD&D hardbacks (dating from the mid-to-late-80s when I ran that system) and a smattering of 2nd ed books (Tome of Magic, Book of Artefacts, some Players' Option stuff - again, with interest as the criterion).
I also have a lot of books for systems that I don't run - including 3E - but that was well-reviewed, or looked like it might have something worthwhile for me to adapt to a game I might run. For instance, I am hoping one day to run some of the Penumbra d20 modules in Burning Wheel - I think it would suit them well, probably better than 3E in many cases.
Needless to say, I am also the GM in my group!