A small solar panel with a charge controller charging a deep-cycle battery that's powering a 12v portable fridge. No need to worry about pulling down the car battery and not using the inverter makes the whole thing more efficient.
For a glass (agree w/Colin on this) display case you could create a variation on what the craft beer world calls a jockey box (basically ice and heat exchanger coil in an esky) and use it to chill air or water to cool a thermal mass that forms the base display case.
It gets more complex Deborah. Maybe an angled tray with glass on top and ice packed on the tray. Then some polystyrene on the ice with the chocolates on the polystyrene so they are separated from the ice. It would stay cool inside and look nice. Angled towards the customers so they can see and also to create a low point to drain the melted water.
You can get sheets of polystyrene from Clark Rubber.
The ice does not need to last all day and indeed it won't but you'll need to experiment a bit.
I suggest glass as opposed to polycarbonate or lexan as it looks good and won't easily scratch. Actually lexan won't scratch either but will craze over time. Glass is heavy and of course easily broken.
You may still get condensation - experiment at home first!
There are commercial displays available too but they are costly, probably heavy and need power.
As regards power, I do all sorts of farmers markets and sometimes I have power, often not. So I use a 1000W pure sine wave inverter and a car battery.
Deborah, I do farmers markets in NSW too. Mine are in Sydney and I think you are "up north".
I find that trying to cool chocolates at the markets is too complicated. You actually have two problems
Keeping the chocolate cool on your stand
Persuading Customers to buy them - they get concerned that they will melt
I cool mine right down before leaving home. I keep them in a cool room at around 16C all week so they are very cool before I leave. I leave them in polystyrene boxes in my cool room so even the boxes are cold to start off.
At the markets I take just a few bags out to display - the rest stay in the boxes (with lids on). I sell these first and re-stock my stand from the boxes. You can use "Lite 'n Easy" boxes or buy them.
Be VERY sure that your stand faces to the east or south. NOT north or west (southern hemisphere). This is CRITICAL! Otherwise the sun will kill your chocolates. If facing to the east you will need to protect it until around 10:00am. Which is OK as people tend not to buy choclate until after 11:00am.
I have used this to work in temperatures up to 38C - I was not happy but I survived.
As regards customers not buying for fear of product melting, it helps if it is cool to the touch but does not fix the problem really. I am about to experiment with making padded metalised bubble wrap bags along the lines of this...