My favorite 2nd hand purchase is Traveller: The New Era. Don't cry foul, just yet, gentle readers. Let me explain why.
I picked this up used, likely at the local rpg shop of the day here in South Bend. It would have been within a year of publication, because I was on Memphis the following year and I had the book before I moved to the South. At the time, the only sci-fi rpg I had played was WEG Star Wars. I had heard of Gamma World, but balked at it after it was described as D&D in space. At the time, I don't think I even owned any of the Star Wars rpg products. I think all I owned was D&D and WoD related.
Anyways, I loved the setting, but the rules did nothing for me. It also fed my developer side, allowing me to create my own worlds. I never got to play it before I moved and I had no idea what people meant when they kept referring to, "Little Black Books."
Flash forward to Memphis. I meet up with a group playing three nights a week and often two games per night. GMs alternated and games came and went. One of the games we played was Traveller. Imagine my excitement at finally getting to play this game...only to find out I had a different edition of the rules. Still, I jumped in without fear and enjoyed the hell out of it. After all, I managed to not die during character creation.
The game was fun and I had a blast with it. There were cool twists and turns. Everyone had a secret - mine was that my skeleton was a form of plastic, not bone, and companies and governments wanted access to me. And I had no idea how I became that way, naturally.
None of that was ever resolved. We moved on to Oriental Adventures before resolution came about. Still, I loved the game so much that I have spent a good chunk of change tracking down nearly all of the first edition LBBs, New Era, T4, and many if the miniature games. I didn't back the new edition, because I have all I need to run the game.