I've been shopping on ThredUp for a couple of years now and have been very happy with all of my purchases. If something doesn't fit I just send it back. I've only had one experience where the item I bought didn't live up to the description. I sent it back and that was that.
I've sent in three bags so far and while I would like to see more money - wouldn't we all? - I'm not dissatisfied with what I have received. I'm happy to get my closet cleaned out.