This is another “gees, how am I supposed to remember” question. Also another “what have you bought” question, boo, hiss, capitalism!
Most of my physical purchases are second hand these days. I don’t want to be the “things cost too much now!” crabby old man but I am. A while back I bought a stack of 5ed Shadowrun books at a deep discount. Those books are about 150 pages and the normal price is $45. That’s bonkers to me. My brain tells me that “should” be a $18 book. But I know that’s because I’m old and I’m remembering how much things cost in 1997. Then again the fact that they were on clearance maybe means I was right?
This isn’t a very interesting answer but probably the “best” secondhand RPG purchase I made was the 3.5 Dungeon Masters’s guide. I believe I picked it up at half-price books for ten bucks. I had been running 3.5 for years without a DMG because I don’t need no stinking book to tell me how to run games but there’s good information in there as long as you’re paying less than half price.
My second-best secondhand buy is probably Enemies and Allies which came out before 3ed edition became 3.5. I’m not normally a fan of “statblock” books because statblocks are the easy part to me, ideas are what I’m after because those are hard, but it had a good balance of NPC stats with a sprinkling of backstory enough to make me like it. It really doesn’t take much to make a character interesting and useless, just a couple of sentences really but most products don’t bother even with that.
Shadowrun deserves honorable mention because I’ve played it a fair deal and I don’t think I’ve ever bought a Shadowrun book that wasn’t secondhand. Which probably means that I’m the worst monster possible because I’ve played a game a lot and the people that made that game got zero money from me. Sorry Catalyst Games. Or whoever owns SR now.
Special dishonorable mention goes to Shadowrun Rigger 5.0. The copy of that book I bought was one where the previous owner had gone through the trouble and expense to put a new cover on it, namely a cover of an anime girl in her anime underwear, which I had to rip off because (anime hate rant deleted). So my physical copy of Rigger 5.0 is a little battered because it has no cover. Even though I TAKE GREAT CARE OF MY BOOKS.
I assumed this new cover action was undertaken because said previous owner was a big anime fan but someone I don’t really believe told me that putting a new cover on a book is part of some theft and reselling scam. I know that in the olden days a lot of books used to say that if the original cover was gone it was stolen and don’t buy it! So maybe it’s true. But I doubt it.
Special different honorable mention goes to Firefly as the game I have all the books for and bought none of them second hand. Actually that’s a lie, I bought a copy of the main book second hand because I wanted extras, but the point is I paid MSP for every book in that lineup.
Which is saying something because I’m a cheap bastard. Sort of. I’m 88% a cheap bastard and 12% someone to makes an impulsive buy and immediately regrets it.