Victoria's not a huge place, so it's not uncommon to bump into people you know. There's only one book store that sells used D&D books (Snowden's... I think it's on Broad Street, for those who know the city). I go there occasionally, hoping to pick up a Dark Sun book - but I've pretty much cleaned them out in that department.
Anyways, I've found numerous little "Goodies" from my purchases. My favourite was when I bought a copy of "A Mighty Fortress" for 2e. Upon opening the book, there was a name in the corner. I knew the guy! What's more, I still hang out with his brother! And, most importantly, *I didn't know he gamed!*. That was kind of cool. If I ever see him around, I'm going to bring it up.
I've also picked up numerous Dark Sun books where the previous owner had highlighted random passages (and they seemed fairly random, trust me). I was quite annoyed at first, but by my third purchase, I had actually come to enjoy "highlight McGee's" notes on Athas. It became a hobby of mine to figure out why one of the chapters of the Veiled Alliance meets at a wine shop, and why he felt it was so necessary to highlight it.
As for character sheets - I've found more than a few. In my copy of Isle of Dread (I picked up in the last six months, mostly out of nostalgia, but also in the hopes of putting a few of the encounters into my Savage Tide campaign), there were NPC notes made in the margins of a few pages - just AC, HP, Damage, and a Name ("Telvenur" or something like that). When I picked up one of the Jakandor books, there was a complete Knorr warrior in the game. However, he had a lot of magical items on him, so I'm not sure whether the group was running a "core" Jakandor game or not!
Finally, when I was young and stupid, I traded a friend of mine a box of Dark Sun stuff in return for his Forgotten Realms stuff (Stupid, I know). He had pages upon pages of crap in there, including an ENTIRE ADVENTURE based around (get this) rescuing a unicorn.
He plays rugby now. One of these days, I want to bring that adventure up, and see how he reacts.