How much competition do you have at the market? Try to grow varities that four other vendors are selling at the same market. Weather permitting plant your corn in plots two weeks apart to insure that you have a good supply of fresh product over a longer period.
We had bad weather for corn last year (frost in June then a moose in the field) but managed to get our third palnting to come in well late in the season...took a few sacks to grill and give to my beef customers as a customer appreciation...one guy asked if I took donations I said sure then he walked around the market munching on a ear and I sold $50.00 worth of corn that I planned on giving away.
As for watermelons...my advise is to grow a variety that is new to the market customers. Up here the yellow ice box variety sells well, and remember......samples sell product, plan on having plenty available for the folks to try.
IMO, avoid the groceries that every backyard gardener grows...we grow pumpkins for the family and sell some of the pie varieties...the rest go to the hens.
Good luck to you...Dave mc