Amazon, if you close your eyes and ignore the egregious fees. You'll still probably get more money than Craigslist and less danger/hassle than Ebay.

Can you sell things on Amazon that they don't sell? E.g., a five year old GPS?

Actually I use craigslist & another local classifieds website. Shipping is "instant", you usually can get lower prices than online, and you can inspect the item, and you have the persons address if they screw you over. It's only really speciality stuff that doesn't sell/can't buy. Also when I sell locally, I usually get a higher price than I would online.

I use tech forums, mainly HardOCP, Anandtech, and Xtremesystems. While most of the stuff bought/sold there are tech items, you can find some non-tech goods there. You might also have better luck buying/selling niche equipment on niche forums, e.g. headphones on head-fi, AV equipment on AVS forums.