Just because you were asked, doesn't mean you should do it. If it is too warm for chocolate, it might not be a good market for you. Even if you can keep the chocolates cool enough to sell, your customers still need to get them home safely.
I seem to remember one guy here saying that he used an insulated cambro like this one with ice in the bottom for his back-stock, and a hotel pan with ice in it to display his individual chocolates.
I'm in the same boat you are. Our first Farmer's Market of the year was yesterday, and it gets REALLY hot (90-105 in the middle of the summer) around here. Fortunately for me, our Market has a large open-air building, so my chocolates were in the shade most of the day. There was a short period that I was a little worried because the sun was directly on my chocolates (shining though a skylight), but I would think that an umbrella would be able to assist in keeping direct sunlight off the packages.
I've been considering picking up something like these reusable ice cubes and wrapping two or three around the package with paper towels to insulate. I think something like that might be sufficient to enable the customers to get their confections home without damage.
Oh, and I'm also considering picking up a small wine fridge and remaking the shelves to fit my needs. They keep the perfect temperature for chocolates, and could even possibly be used as a display fridge if needed...
Wine fridges can only help if the ambiant air is +/- 15' of your desired temp. I can tell you from experience you will freeze up the unit. Not to mention every time you open the door you flush the unit and those units do not have the return capacity to make it nice again. Condensation will also quickly happen and then you'll have water droplets on your product. Experiment, it's the only way to really learn, but I just finished toting out two of our coolers from those days. ;)
These type of questions come up each season. Search around the forums for a lot of solutions:
My advice and you'll find it in one of those threads. Make fakes or sacrifices to the heat for your table, keep multiple coolers on ice (dry ice is too cold), use sealed containers containing product and multiple at that.
Your enemies are heat and moisture. Heat melts/warps, and condensation from constant cooler access will eventually create condensation on your products. If you limit the exposure or access points you can have healthy market life. We've sold at farmers markets, multiple, for the last 5 years in the Carolinas. Now we have the luxury of taking off July and August due to having a shop but we know how it goes.
Much luck to ya!