It has to happen to everyone above the 30th parallel.
The temps drop, customers dwindle but you still have stuff to sell.
So what do you do?
All the markets I sell at end before it gets too cold (or too wet) Try advertising a special deal? If you have a local gardening show, ask them to plug the market and that this is the best time to buy / plant / etc. If no garden show, get local TV or radio stations to plug the market.
See if some of the craft shows or festivals might let you be a vendor at a very reduced rate because you are a farm. Some around our area do that in the fall.
I am still doing markets and the dark cool days have certainly lessened the amount of people coming out but we are still selling a lot of food.
the markets end soon so I am finding while we have fewer people they are buying a lot more. I think they are stocking up a bit for winter.
Most winters we do sell off season things like garlic, squashes, onions, carrots, etc., I have an email list and email customers telling them what we have and how to get it.
I also take food into a couple of health food stores that I have been doing business with for years and that support local foods.
if you belong to a church ask your fellow members if they would want to buy food from you. not a church goer? Than take a cooler of food to all meetings you go to, tell folks you have things for sale and you will likely sell out the first time. Take orders for the next meeting . Everybody has to eat and most appreciate fresh local food.
Be creative and you will find you have the beginning of a good out of season market.
I have found that there are two kinds of market goers. The "regulars" and the "sight-seers."
The first will be there in a thunderstorm in 45 degree weather and will buy a good amount every time. The others hardly ever buy anything.
You can here the sight seers say things like "Wow, its the same price as the grocrey store!" I want to wack them and scream "TRY IT! Its a heirloom that tastes fantasitic and it was picked less then 24 hours ago, so all the enzymes and nutrients have not started to die yet!" They carry a bag with mostly baked goods and crafts, a $2 cookie makes more since to them then a $.50 zuccinni or $3/# heirloom toms.
But the regulars are always there and they will buy until the last day of market. These are your gold! We actually have diverted a few of these people to our CSA. But these are your late season buyers, so try to get your own regulars as early as possible in the season and it will help you later. (Typically there are also fewer farmers after first frost, you can grab their regulars to, and if you are there early enough the next year, maybe make them your regulars the next year.)