You can sell meat on your farm as long as you sell under 1,000 birds. (this applies to like 30-40 states).
Those states that don't abide by these rules use federal laws. Which it states you can process up to 20,000 birds on your farm without having a facility.
Now if you have a state inspected facility. (which all it has to be is a on farm processing site that will meet sanitation requirements.) Basically someone will come in and make sure your equipment and processing tables are sanitary. They may go as far as testing your vats/buckets or even your birds that you process. If you have this done you can raise 20,000 birds with the exemption.
I've never sold at farmers markets but I'm almost positive they have to be USDA processed to be sold at a farmers market in most states (don't quote me though). Greyfields do when you take your birds to the processor.. it's not a USDA inspected facility is it? Probably just state inspected... How does this exempt you to take to farmers markets?
It's better for me to stay under the exemption by selling on farm. But for many the location of their farm is not in the mix of things. For me I'm with in 15 minutes of 3 farmers markets in the area, so I really havn't looked into it as the market is already here.