ReadWriteShop is an occasional series about the intersection of technology and commerce. Today's women's fashion "re-commerce" websites are certainly no bargain bins. Sites like Threadflip, Poshmark, Bib + Tuck, LikeTwice, and Thredup are re-selling used—sorry, make that "pre-worn"—haute couture, and even pieces from upscale outlets such as J. Crew, like nobody's business.