I do a couple things to keep my fresh cut herbs looking fresh at our farmer's market:
1. I cut them right before I head to the market after everything else is loaded into the car.
2. For things like oregano, mint, chives, etc, I treat them like flowers and put them in a vase with some water. For Basil, thyme, sage, etc, I keep it out of the direct sunlight and will sprits it with water occasionally.
3. Alway, always, always have some receipes on hand that you can share with customers. Maybe it is just my area, but I talk to a ton of people that say "oh, I don't know how to cook with fresh herbs." lol
4. I do sell quite a lot of herbs to area chefs. That has actually been very good for me.
I don't sell herb plants. I want them to keep coming to me rather then growing their own.
Good luck! I love growing the herbs and selling them