Here’s what consumers have asked with answers from Threadup staff and previous consumers.
It may be "legit" but they have terrible business practices towards sellers. They will basically TAKE what you send in an not give you any credit for it, nor give you a good reason why it was not deemed "acceptable." They basically took a nearly new Longchamp purse (used ONCE-not my style) and tried saying it wasn't acceptable. Meanwhile, they have like 6 of them for sale on their site for $60.