ask for a sales tax exempt form. or say, "hey i'm a farmer and I qualify to buy things without paying tax, so don't charge me." cashier: "well you have to fill out one of these forms, sir."
the form will probably require a tax number so if you don't have one you may be out of luck.
but to be honest with you I don't know anything about those laws in FLA and am talking about what you would do in VA.