This feature is available on the web only (https://gameflip.com/)
- Go to Listings/On Sale https://gameflip.com/listings/onsale
- Select all skins you want to return and click on "Take off Sale"
- Go to Listings/Draft https://gameflip.com/listings/draft
- Select the same skins and click on "Return Skins"
- Now log in to Steam and accept all incoming trades from the Gameflip Bots