So I go down to the local Salvo's (Salvation Army store for all of you who don't know them)
I look through the CDs, and find THE NEVERHOOD, for $2, a very old and rare game
not only that, but TOTAL ANNIHILATION, fo $2 also
Turned out, with some vinyls I got it reduced to $6
Then I'm home now, tested them, and THEY WORK
WOW

HOLY CARP. Man I've been looking for a copy of the Neverworld in my local charity shops for years! Great luck dude!

Good Luck!
I came upon it in a broken case
I've been playing it , and it's a GREAT game

At my local charity I managed to come across a mint copy of Sanitarium for £1. Didn't buy it though. I thought that a GOG copy would go down a treat instead of having to do some technical trickeries.
Although if it is still there next time I go I might as well get it.

I will say however I bought a boxed Megadrive II with all cables, two controllers and 3 (crappy) games for �2.50 at my local charity shop. They couldn't sell it as it was electrical, so I took it off their hands for a donation :)
(for Americans, the Megadrive is a Genesis)

Vinyls. Hah. That's about as modern as my dinosaur breeding ground out back.
Electricity is used for terrorism so if you sell electric things then YOU'RE FUNDING TERRORISM

Health and Safety, charity shops are unable to sell second hand electrical items just in case they kill someone.
edit: quoting cock up.

I just spent $25 for a big box copy of the core contingency of ebay and the guy destroyed the box to save on postage which ended up costing $2.75 and he charged $10.

Not as nice as your find, but I managed to snag a copy of The Last of the Mohicans at my University's open house fair just now.

I hate second-hand stores and PC games because they are always awesome games for awesome prices but my collector brain will not buy them because they are never complete.
So I pay $30+ on ebay.

I like my retro mood swings, leav eme alone!

Health and safety I think, so that if it kills someone due to dodgy wiring then they can't get blamed probably (edit already answered and the quotes mucked up) Just went to a charity shop and picked up Duke Nukem 3D, Magic Carpet and Resident Evil 3 (which I have no idea if it is any good). They also had Panzer Dragoon (which I didn't buy because I heard bad things about the PC port) and Mortal Kombat II (which I didn't buy for the same reason). Sadly, Magic Carpet did not come with the 3D glasses as advertised on the box.

Yeah, I love Good Will's and the like...
Picked up my N64 for $7 from a Good Will.
They thought it was broken, I replaced the AV cord and BAM, works like a charm.

I still play my N64, SNES, and even the failure that is my Dreamcast....
That being said, I did manage to find my old Slave Zero disk....so yay me.