Half.com is a subsidiary of eBay, in which sellers offer items at fixed prices, usually items that have a UPC, ISBN or other kind of SKU, rather than rare, old or collectible items. The items available on half.com are limited to books, textbooks, music, … see full wiki
When it comes to buying and selling books, CD and other media, half.com is the first site that I visit.
On the buying front, I love the easy browsing. I can compare the prices of different sellers and different conditions of the products, and when I've found something that I want to purchase, once my information is on file, I can just use Click-n-Ship each time after that. No need to reenter in my address and credit card number anymore.
On the selling front, I dig half.com because I can sell my goods at the price I want, take it or leave it; it's not an auction or make-a-deal site. They also do not charge a fee for listings, though they do take a small percentage of the sale once the item is sold. And my two most favorite things about selling on half.com: they give sellers a small shipping allowance to send items through media mail, and listings don't have an expiration date. I can keep my listing up till it sells. Another cool thing about half.com: once you find your item and list the condition it's in, it tells you what the last item of your kind sold for and what its average selling price is.
Half.com has saved me so much money on college text books. It is one of many reasons I love the internet age.