I love the Spring Chicken sale... and it's coming up next month! I got my son some great outfits there last year. http://www.springchickensale.com/
Kid-to-Kid is very picky. And I don't think they give a whole lot for what they buy. They definitely make a good profit on the stuff when they resell it. Much rather buy from there than sell.
I've sold on Craigslist before and had good luck... I think it's more for big items though.
Also, there is www.kizoodle.com which is like Ebay but you get Kizoodle cash instead of real cash (so you pretty much swap with other people)... they have all sorts of stuff. I have a code for them. If you enter referral code 7JT1PN they were giving you free Kizoodle cash