Crochet is by far my favorite craft. It is so versatile, one can make almost anything using a hook. It's no wonder that crochet in its many varieties can be found at craft fairs.
My primary medium of choice is 100% wool. I live in an area where I can purchase this yarn from local farmer's markets at an extremely reasonable price. People are willing to pay more for items made with wool than with acrylic. My best sellers at craft fairs are by far hats, mittens, and gloves. One year I made 30 pair of fingerless gloves in various sizes, and sold every one of them between the 2 craft fairs I attended.
Hats, mittens, and gloves are also popular items for knitters and fleece sewers. Therefore, I try to have some unique crochet items on my table as well - thread bookmarks, different styled wool hats, doll accessories that match child size accessories (ponchos, purses, etc.)
Sweaters are also generally a good seller at craft fairs, if you have the time to make them. Remember you should have a variety of colors and sizes for people to choose from. The days of having a sample for people to look at and order from are pretty much history. Customers want to be able to take home the items right then. The only exception I have found for this are items that are best when custom made: Christening gowns, dresses, and bathing suits.
Anything that is unusual or looks elegant generally sells well if it is priced well. Think crocheted lace, angels, Christmas ornaments, snow villages - all wonderful choices for fall shows.
When deciding what to sell at craft fairs, you need to keep in mind how much time it will take to make, material cost, and what you can reasonably sell it for. Cute little cell phone cozies make only take an hour and very little yarn, but they don't sell well. Electronics do not like to be kept warm. It may take you 8 hours to make a hat, but don't expect to sell it for $80.
The more craft fairs you attend, the more you will get a feel for what items sell and at what price. If you talk to other vendors and customers, you can get a better idea of what would be a good seller.