Look at what I found at a yard sale today.
Apologies for the poor picture quality, I had to use a webcam to take them. Anyway, I got the first DM guide and the Player's Handbook during the first visit. We went back a few hours later after everyone had finished setting up their tables and I found a second copy of the former and a module, Dwellers of the Forbidden City. The mindblowing thing is how little I paid for them - the guide and handbook only cost six bucks total (he had them marked eight apiece), while the second guide and the module were but fifty cents each.
I thought the two Dungeon Masters Guides were identical at first, but as you can probably see in the first picture, there's a slight differences with the covers, mostly with the TSR's logos and name - the first copy has TSR Games, while the second has TSR Hobbies, Inc. I have no idea when the guides and handbook were printed other than 1979-1981. They're definitely for 1E AD&D.
I'm considering doing a contest in the future with one of the guides as the prize.
I would've failed my saving throw if that guy had had the Monster Manual too.