I don't sell cookies--YET! (due to lack of a cottage law in my state)--but I am thinking ahead about displays for farmer's markets/events, too. In addition to tabletop displays using Styrofoam rectangles glued together as "steps" and draped with fabric, I was thinking about using a stand-up artist easel or two and designing a light-weight rack to place onto it. I think that foam board with one-inch construction spacer slats glued onto it would hold a bunch of cookies. It would lean back against the easel so gravity would hold the cookies upright without them falling off forward, and stoppers at the end of the slats would hold them in at the sides of the foam boards.
These are simply ideas at this point (except the Styrofoam Steps, which I have used at showers and parties for family and friends.)
You can pretty much get any supplies you need to make these things at AC Moore, Hobby Lobby, Michael's, JoAnn's, or most other art supply stores.