Y.M.
Around here an 8 oz jar is priced at $6.00.
Jelly/jam/preserves don't seem to be great sellers in this area (though I bought some delicious apple cinnamon jelly last year). People go to the farmers market for produce and baked goods (made by rthe local Amish...the pies go quickly) and kind of ignore the other tables. Hopefully it is different in your area. Good luck.
Eta: We have some tables that I think are just stupid and more suitable to a flea market or craft fair. I don't know if it is just me but I don't like seeing the farmers market filled with people hawking Tupperware, overpriced children's books, knick knacks, etc.