I have sold books on both Amazon and half.com . Amazon gets more traffic, so things tend to sell faster, but half.com is easier and more straightforward to deal with. I prefer half.com .
One problem I ran into on Amazon was customer responses. I had one who gave me a bad rating for somebody else's book (not one I had ever listed); must have confused the two sellers. Another one tried to leave me the highest rating (judging by his comment) but misunderstood the rating system and gave me 1 star instead of 5. I left comments on those ratings, but I didn't sell many items on Amazon so these made a huge dent in my %-rating. I myself will skip over listings where the person has less than a 90% rating, so I just figured I was done with Amazon.