I used to live in Sweden, which was a desert for retro Mac stuff. Now I've moved to Japan though.
HARD OFF is a chain of used hardware stores here in Japan. They have a "Junk" section which is full of untested or broken stuff. This is where I like to go if i have some spare time, if only to look for things I never knew existed.
I plan to add interesting stuff I spot here in the future, but for now, check out the imgur album I uploaded once of a random visit to HARD OFF
If you have something less generic than stick of RAM or a DVD drive in mind, Yahoo! Auctions is your best bet. Don't forget to check previously concluded auctions for an idea of what the real value of an item is though - sometimes mundane items can get bid up to absurd prices.
You can find a lot of the same stuff you can find on Yahoo! Auctions much cheaper on Mercari. My theory the sellers there are more "normies" who don't realize how much people will pay for this old junk.