I grow a few extra tomato (5 varieties), bell pepper (two varieties), onion (yellow and purple), and garlic and Italian chive plants to sell during the spring. The only reason I do so is because when I buy the seeds/slips, I end up with so many I have no place to plant them. I don't sell them at any market though, but by word-of-mouth, people contact me and come to the house to purchase them. I only charge $5.00/1 gal. plant.
I grow a lot of garlic and in fact, it is posted on the "Co-Op/Classifieds" forum. I posted them "pre-harvest". I sold out completely last spring, and in fact had to greatly restrict and even cancel a lot of orders. I am doing it differently this year by allowing people to go ahead and lock in what they want. I don't want payment though, not until I actually dig the garlic up. I grow organically, and all the garlic is considered gourmet, heirloom. The flavor and pungency of this garlic will knock your socks off!