Great thread
There is a church that hold farmer's markets in the summer and all it costs to put up a booth is a donation to the church ($10 is customary).
I thought about selling plants last year. Turns out a LOT of other people had the same idea. So scratch that...LOL
I have wanted to offer fresh cut herbs. Basil is popular at the big farmer's market down town. It would be worth a try since they don't take up a lot of room and I have a LOT of herbs in my garden.
I should have plenty of quail and duck eggs for sale. Possibly some hatchlings too.
Pumpkins and squash do well in my area. I am planting butternut squash so i can sell extras. I also seem to have a knack for growing dragon tongue beans. I have never seem those at our farmer's markets. Those could be really fun for people to buy, and if they don't sell, I have no problem eating them myself. (The ducks will also help if I ask them

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I had a bumper crop of grapes last year so maybe I can sell grapes.
Tomatoes sell well in my area. Well, anything required to make salsa sells really well

I have not been able to grow enough for my personal consumption yet, but if I had extra, I know I could sell those. Same with jalapenos or other peppers.
Zucchini sell well...mainly because people don't want to give up garden space to grow them and they are sold pretty cheap.
Carrots...not sure. I never bought any. If there was an exotic variety I couldn't get at the store I probably would. They are hard to grow here because of the hard ground. Green beans went very well at the big farmer's market. I know I bought a lot before mine were ready
Melons, berries and peaches sell well here and command a good price. I am planting some Collective Farm Woman melons, ground cherries and alpine strawberries. Surplus can go to the market, but I doubt there will be much extra...hee hee hee
I also have 2 apple trees. A late frost killed all the blooms last spring, but the trees are at the age where they should produce a lot of apples weather permitting.
Basically anything I would be selling is something I really like for myself. If I don't sell a single thing, I still have a personal use for it, can freeze or preserve it, or trade it to friends.