I can only tell you what I have experienced at Flea Markets, Fairs, Trade Shows, and other places that you do not have ready access to all of your Utilities that we usually take for granted.
If you wish to delve into this type of selling, do not attempt to do it Half Way. When envisioning and building your booth, think of your space as a Cube and try to sketch down how you will envision it to be, when completed.
Start by defining your space.
Are you going to use a single space that most buy, of a 10'x10'X 10. 10' wide or side to side
10' Deep or front to back, and 10' Tall. (or a Double Space of 20' wide)
BUY A CANOPY, you will be closed down, more than open, if you don't have a way to keep the sun off of your head, when it's nice, but more importantly, the Rain off of your head, your beans or equipment, when it rains.
Remember the floor as well, you will thank your stars, if you stand on a cheap, foam mat instead of concrete, or even a scrap piece of AstroTurf to keep your feet out of water and mud when it rains.
Be aware, no matter what size booth that you have, it must look Full.
Anything that you can do, in the morning when you open, You must look Full. Be there 1-2 hours before the promoter tells you to be there, because if you are, you'll probably get patronized by the other Vendors, when they smell the coffee.
You can, a lot of times, make your booth rent back, before anyone else is open, just by selling to the vendors.
Be Neighborly,
give away a cup of brewed coffee, to the other vendors. Remember, a pound of ground coffee can make 40 k-Cups or brew in a drip maker to 30-40 cups, with your cost, brewed coffee should cost you if you used, bottled water is .06 and if your cost for the ground beans should cost you no more than $6.00 a lb, so the coffee would cost you .15 a cup, the cups .05, so your per cup cost should be .26, if you give away 20 cups, in the morning, and you sell to the vendors, cheaper or a discount, for like a buck a cup.
This $5.00 can be written off as a Promotional Advertising Expense, when you give it away, you write off, the retail as a promotional expense so if you give away 20 cups, you may be able to write off, your retail price, which may be $2.00 per cup and may be as high as a $40.00 write off, so for a little over a $5.00 investment, you build goodwill, you gain a customer that at least casually will buy from you. And you may be able to write off $40.00, but check with your tax professional, to decide how much that you can actually, write off. I get Mugs Printed or Cups that have my logo on them. So while your customers are walking around, they see all of the vendors drinking YOUR, coffee, in your cups. Don't give them cream & sugar, make them walk to your booth to dress their own cup. It will show that the other vendors, like your product. And are your customers, so there is an immediate trust implied. In the mind of your Customers. I get Plastic Cups or Mugs made up to sell in the store, but also to give away. The cheapest thing that you can give away is a Beverage, so I get all of my neighbors, to carry my businesses mugs around and let's say I had a sandwich place, they could get a beverage for free, when they bought a sandwich, everyone that was in the mall saw that person from another store, walking to my store carrying a Logo'd Cup.
You have to build and tear down daily, your booth, so make sure that you keep your Tables and Display Pieces, manageable. When I owned a Food Truck, we had similar concerns, about power, so we used a Honda, Quiet Generator. That's the only brand the guys that own food trucks around here, use. The quiet ones are more expensive, but listen to that really loud, $300.00 generator for one day and you'll be ready to rip your ears off. Make sure when deciding on a size for your generator, you take into account, everything that you will need to be plugging in. If you just want to run a Brewer and a Grinder that is 110V, takes a lot less juice, than if you have to run several 220V, Grinders, a Roaster with an Afterburner, a Cash Register, and don't forget about lights. The LED bulbs out on the market have very low wattage requirements, and at Home Depot, they sell LED bulbs starting at $2.50. Just write down EVERY THING, that you will plug in, tell the salesman and add at least 50% more, for expansion power needs.
if you want to Roast there, I would only consider a Propane Roaster, because those large tanks last a long time and the energy conversion difference in hauling around the extra fuel that you would need, to run an Electric Roaster, would not be worth it.
Consider your Climate as well, does it rain a lot, in which case you want to make sure that you have an awning, for potential customers, to come in from the rain, or are you going to have an Open Entrance, that they could walk into to middle of the space, before they get to order? You will also need to make sure that your canopy has walls to keep the blowing rain, from your beans and equipment.
Your design will Depend on if you are going to be selling cups of coffee, or Whole & Ground Beans.
If you are going to be selling Prepared Coffee, are you selling Snacks & Pastries? If you handle unwrapped food products, you need to check with the Health Dept, and see what requirements, that they have. At minimum you will need a wash station, with 3 sinks or containers, and a Hand Wash Sinks. Theses can be done cheaply with Bus Tubs or you can do what serious operators do and that is buy or build a Self Contained sink Unit. I Built Mine, (cost was around $800). I can give you a link to the plans and material list, that I built, if you'd like? Or you can look for a Used Unit, new a Stainless Steel model, with pumps and heaters, can run $5,000.00. Used, you can usually find one in the $2,000-$3,000, range. Hand wash stations and sink units can be rented temporarily from the folks that rent Porta-Poties, but the expense is high, unless it is a one time thing.
Storing and transporting your water, that you will use to brew cups of coffee, prior to brewing, must be in a Food Safe Container and should be filtered before and after going into that vessel. Beforehand, I use a inline filter at my Commissary or if working out of your home, you can get a small table top filter unit cheaply, online and then right before you brew, with it, you can run it through a Britta Filter, or something similar. Remember if Water is the single largest ingredient, in your Coffee, it must be Crystal Clear and Devoid of any Tastes or Odors. I use a 6 gallon Plastic Carboy and a hand pump on top to dispense it. If that will not work, you can buy bottled gallons of Spring Water, you can find them for .99 a gallon. You can buy those Carboy vessels and pumps at any store that sells Beer and Winemaking Supplies.
I hope that answered, most of your questions. If you have any other questions, ask it here, so that others may learn as well.
Good Luck
Chef David