I hit up the used bookstore again yesterday.
I ended up adding Assassin Mountain, a boxed set for Al-Qadim, to my collection. I've heard good things about it, and it was written by Wolfgang Bauer, whose works I have enjoyed in the past. Plus it was less than seven bucks.
Also there was White Wolf's rare and out of print Street Fighter roleplaying game, but the idea was too ridiculous, even for me. (The artwork was also terrible, so it would have added nothing to my collection anyway.)
There was a ridiculously cheap set of GURPS 4th edition core books, and I did actually consider them for a moment...but then I flipped them open and remembered why I hate GURPS so much.