I've had two good buying experiences with ThreadUp so far. The clothes are in great condition (read the provided descriptions for any notes on condition), are as described, and are packaged beautifully. Prices are fair and the website is well designed and has the clearest photos I've seen of the clothing among all of the online resale sights I frequent. I've never sold any clothes through ThreadUp, so I can't comment on that aspect of the site.
The best part is you can get a free $10 towards your first purchase at ThredUp!: http://www.thredup.com/r/ZFCJYY