For one, rethink lettuce in 90 degree weather. It will bolt and tipburn much over 86 degrees unless you grow it in partial shade, then it will be lanky. Slugs are a big problem with lettuce. In fact lettuce makes good slug bait. You will need to use slug bait to control them. Spinach likewise is a cool weather crop unless you are growing NZ spinach.
The most heat tolerant lettuce will be the red lettuce, butterhead, muir lettuce, but you will have to pick them young, if they start to head up they tip burn and bolt and they start to turn bitter before then. You will have to spray weekly with neem or insecticidal soap to keep the thrips, aphids, and mites under control. They can get through most of the insect netting and even green houses.
It helps to know where you are so update your profile with your location and zone. Lettuce and other cool weather crops are more in demand in summer mainly because they are harder to get. Farmers will have high losses associated with environmental issues like storm damage, tipburn and bolting.
If you are new at farmers market, I suggest before you dive in, you should do some market research. At my farmer's market, I notice everybody has almost the same thing at the same price, so you have to make what you have stand out by being of better quality. Don't try to be too diffferent people like purple eggplant but have a thing about the color green. It takes some education to get them to try new thigs. so be prepared to sample and pass out recipes.
Things that are more suitable for growing in the summer and good for the market are Japanese cucumbers, beets (they are a cool season crop but tolerate warmer weather better than lettuce if you give it enough water, tomatoes, eggplant, beans (they are a pain to harvest though and have a short shelf life. Herbs and fruits are good. If you live where I live in the tropics papaya is ready to harvest in about six months and produces for a few years, so do eggplant, hot peppers (sell them fresh or dried). Jalapenos are cheap at $2 a pound but small hot peppers go from $6-$12 a pound. If you can grow good basil you can sell that to restaurants. Cherry tomatoes are more productive and have fewer problems than larger tomatoes. If you have large local ethnic groups you can cater to that with daikon radish, tomatillos, citrus fruit, bananas, tapioca, lemon grass, leeks, coconuts, or gourds.
Things that are cheap and common are not worth while like carrots and corn unless you have a lot of space and you are going for the specialty market, they don't fetch a high price.
I think you need to do your homework first because depending on where you are you may need a license, vendor insurance, and may need to meet specific requirements depending on the regulation and rules of the market. There are also vendor fees as well as getting to know what your customers want. It may take a while to build up a customer base so be prepared to be in it for the long haul.
You also have to figure out what it cost you. Would you be willing to work for less than a dollar an hour? Don't quit your day job yet! You have to figure out if you have the right product mix and where your break even point is. If you don't bring enough to the market to make enough to even get to the break even point, why bother?
http://www.agr.georgia.gov/Data/Sites/1 ... -Guide.pdf
http://www.hobbyfarms.com/8-tips-for-be ... vendors-4/
Happy gardening in Hawaii. Gardens are where people grow.