I have bought from ThredUp on 3-4 occasions and was happy with my purchases. They don't do a very good job at describing the items, such as fit, material, dry clean, etc. it is hit or miss. I bought a Gap jean jacket that was in like new condition but it had a heavy fleece lining that was not in the description. I contacted customer service, they were very cooperative and waived the return shipping fee. I don't have any complaints about customer service.
Selling clothes to ThredUp is another matter. They buy low (really low) and sell high. Also, their criteria on accepting or rejecting is not consistent. I sent brand name clothes and shoes in excellent condition and was told that some of the items didn't meet their standards. Yes, I could pay $12.95 to have them returned but really, it's not worth paying to get back what I already decided I don't want anymore, at least not at that price. I, too, wondered what they do the clothes they decide not to sell.