Unless you make things in a kitchen that is not certified you don't really have to have a license. Sometimes a nice cold soda or water, etc, will keep the kids happy while the parents shop. I have seen folks pay as little as .18 for each can and sell them for .50 which is a nice profit just to bring it home.
I would stay away from anything you have to bake or make in any way. I have seen it backfire on folks who did nothing wrong, but paid a fine anyways.
Why would you want to sell food at a garage sale? But for pricing you are going to have to figure out what it cost you, ingredient and time wise, then add whatever to the cost so that you make a little profit. I think it sounds like a lot of extra work. For one thing people will be asking for drinks and food items that you don't have.
You might want to check and see if you are allowed to do this without necessary licensing.
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