I've used thredUp on both ends of the spectrum, as a consignor and a shopper. Both experiences were easy breezy. As a consignor, I love that I just have to request a cleanout bag, stuff it with all my gently loved garments and accessories, then send it back.
As a shopper, I love that I can read product descriptions, place an order like I do on any other online retailer site, try the pieces on when they arrive, and send back what doesn't work for me. Super simple and super affordable!