Last year I started a special section for direct sales only. The area in which I grow this stuff is set off from the shareholder area, and although it contains a lot of the same stuff, also produces some unique crops.
To start I went to the grocery stores in my area and figured out what produce, besides fruit, was fairly expensive to buy. Then I took that list and decided of those which I could produce in semi abundance in my garden.
What is expensive in your area may be different than in mine, but you get the idea. Next figure out how much you can sell a set quantity of each particular vegetable for. You should be able to easily beat the store price. If not, then you may have to resort to more of a "gourmet" type version of the veggie.
Before you even plant a seed in the ground, make a list of who would buy produce in abundance. I found that great niche in small cafes and, crazy as it may seem, seniors housing. Explain your product and your price. Leave a price quote. Tell them you will bring a sample as soon as you pull some out of the ground. Let them know that you can keep them in whatever it is for the entire summer. Finally, I usually offer a set price for the summer.
And then the biggest business builder of all:
Under promise and over deliver!