I've now bought several items on ThredUp, and kept the majority of them. Clothes are truly as described--like-new clothes really are just like new. Packaging is tof good quality, and double-wrapped to keep clothes safe from water or other damage in transit. The color values on the website are close to true. The only problem I've had is sizing. It's better to go by the stated size than by measurements, I've found, as the waist and bust measurements are from I-don't-know-what. Naturally, sizes vary--a "medium" or "12" from one company can differ from another, but it's still more accurate than the waist and bust sizes. For example, I recently bought a size large blouse that was listed as having about a 36 inch bust but it was actually slightly large on my nearly-size-14 frame and fit my 40 inch bust perfectly.
I hope ThredUp can work out issues with measurements because otherwise it's a fantastic source for secondhand clothes!