The Tomato is the #1 garden plant in the US.
I'm agreeing with
@Artichoke Lover , peppers are also a good choice. Be aware that other than bells, you might have customers with very specific wants. Since those may vary, it's best not to have lots of your favorite and zero of everything else. A good mix is likely to attract attention - of course, that means lots of signs and plant markers.
Lettuce is for you

. You want to continue selling veggies during the growing season - salad vegetables have the highest value per square foot. You need consistent customer purchases, tho. The salad vegetables are all perishable and it's not like you can bring them back next week.
If you are planning to be doing the selling for weeks and weeks, perennial herbs can be brought back, again and again. You may not have that many customers for something like rosemary but they are likely willing to pay a higher price.
Flower starts, you are at a disadvantage. Of course, if this is a high traffic farmers' market, you can grow them for cut flowers and get some of that valuable square feet from your garden. The disadvantage you have with the starts is competition with the big outfits. American consumers are used to buying plants with flowers already on them. That will be difficult for you unless you adopt the commercial practice of spraying them with growth retardant. Yep, they require maturity to come into flower but if you want to sell them in 4-pacs, 2" pots, even 4" pots - they have to mature while their growth is restricted. You may have problems with growing space and transportation but planters filled with a variety of flowering plants may make sense. Still, the consumer is spoiled into expecting beautiful flowers from the git go.
Steve