I've not had any real bad experiences with websites that I'd say to avoid (I did have a really bad one with Amazon, but that was with a third party merchant that sold through Amazon that has since been removed), nor really had any outstanding experiences that are worth recommending a sight.

The one thing I do have as a recommendation, if you are looking for a good way to help charity with minimal extra effort, I'd suggest going to goodsearch.com and using the shopping feature... they provide links to lots of online stores that will donate a portion of the proceeds from your sale to charities of your choosing.