I check my "personal shops" every day. This feature that allows me to save my favorite brands and sizes is really, really awesome. Why can't Nordstrom notify me when there is a new Tahari jacket in my size on their site? Well, ThredUp can and does. I have scored some very expensive clothes at 20% of the retail cost. (Note that the clothes that were cheaper to begin with are not as deeply discounted.). I've scored some good kids clothes and shoes too.
I don't love ThredUp as much for selling, but I credit it with motivating me to clean out my closet.
Shipping terms and returns are great.