I have been selling/buying from thredUp since it's beta times, and I have been really happy with my transactions. Although, as people have said, sometimes your pieces aren't valued for as much as you'd hope, it really depends on what they are looking for at that moment. If an item has a good label, if it isn't currently in fashionable, it won't carry a high price tag for you.
That being said, I have an great luck with both selling and buying. Some items have brought in better return than others, but I love that I can upcycle my clothing, and get discards from others that are exactly what I was looking for. I like to online shop as a general rule, so the ease of this whole process is bliss. I have sent loads of bags and spent hundreds of dollars that I have earned and gotten great pieces, both statement and basic pieces, for myself and my daughter. The children's designer stuff have the best deals for sure.
I will continue to use them and recommend to others! Now if only they would start accepting men's clothing...!