I have had mediocre success shopping on Thredup.com; one order was well worn despite being marked as "like new," but two others were more on par to ratings. In the interest of time/energy I did not make a formal complaint or return the item. In terms of pricing, I have noticed sales at Kohls, Marshalls, and Nordstrom Rack of new items are often cheaper than the identical used products available. This deters me from shopping on the site more frequently. Again, with small children running around the house the notion of online shopping is certainly tempting, but it is not necessarily saving me money on quality items.
In terms of selling, I have mixed feelings about my experience sending in a bag. I am thrilled clothing is no longer sitting in my closet. However, the amount I received for the clothing I sent in was a fraction of what I would have likely gotten at the consignment store down the street from my home. It was the convenience of mailing it that lured me, and not having to follow-up with collecting cash etc. I was happy to have "store credit" towards additional items.