It's fun to pick out clothes from this site. My oldest is a teenage girl, so she really enjoyed that. But sometimes items are not in the correct category. For example, I ordered several school shirts for my boys and almost half were actually for girls (they buttoned the other way). It was annoying, but ThredUp paid the return shipping on them and refunded me.
It would also be helpful if dresses and skirts had a length listed, some of them do and some of them don't so it's hard to know how they'll fit.
I have had success as a seller too. I've sent in 3 bags so far, 2 have been processed already and I'm happy with the amount they paid. You do have to be very careful to check which brands are accepted by them or you'll be disappointed. And keep in mind the actual processing of a bag takes a long time. It takes over 2 weeks to get to their warehouse (we live in NY and they're in CA) and another 2 weeks for them to go through it. So you need patience.