For me there have been three things that make yard sales work:

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-good advertising. I find a way to list *what* we have on the posters and emails. A lot of people are looking for specific things: baby clothes, books, antiques, and you want to get the right ones to your sale
-really good signage. I live in a dense area with confusing streets. I print out three types of big arrows, and in smaller type have the address and date , and I go wild putting them up in the neighborhood (and try to find and take them all down later)
-really low prices that get lower as the day goes on. My goal is to bring NOTHING inside. If I think I can sell something for double digits, it doesn’t go in the yard sale, unless it’s one of the things that people look for (around here that would be good wood furniture, bicycles, large baby items) I’ve made money on yard sales in bulk. Which also means, can’t have them too often.