If you have clothes please hang them or fold on a display table it's difficult and unappealing to go through a box of stuff and on hangers you can get more for the clothes.
Have lots of $1 bills and lots of change if you are selling stuff for change or at least quarters.
Bring lots of food and beverages for your family the concessions are often crappy and overpriced.
Put a price on stuff a little higher maybe depending on the item up to 10-20% higher so they can negotiate it down and feel like they really got a bargain.
Put some free stuff in a basket with one item for each $5 or $10 or whatever (depending on what you're selling) purchase.
Make sure things are displayed pleasingly and everything is grouped together so people will grab lots of tools or shoes or fishing stuff.
Be patient and watchful and don't let anyone take advantage of you. A smile and a joke go a long way. You'll be tired at the end of the day so have a dinner plan already and possibly a hot bath waiting!
Have fun and make money.

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The political system is broke and it's a joke.