I am looking for a pricing guide for used Apple laptops, desktops, and iOS devices. Does anyone have a recommendation on where to go to find a guide like this? Preferably a free guide.
http://www.everymac.com has estimated current price for every model (It's at the bottom of the table). I'm not sure where they are getting those from though.
Check out Gazelle. They buy Apple products, as well as many other consumer electronics.
For the most recent price for a particular model in some particular condition, I try to find a bunch of completed sales on eBay with best matching descriptions and pick the median closing bid price.
Noting the variance in prices is also important, since used devices don't really have any fixed price. It depends on the buyer, seller, location and date (phase-of-the-moon, last poker hand, etc.). YMMV.
I would suggest BuyBackWorld to get the cash trade in value of your Apple products. Their Apple buyback program lets you sell iPhones, iPads, Macbooks, etc to them instantly.
Try eBay or check out Green Star Tronics.