In my experience, people who hold garage sales but never go to them don't do as well. They often don't put up adequate signage, mark visible and reasonable prices, offer the kinds of things that will sell well, etc. As a veteran garage sale customer, these kinds of gaffes are immediately apparent to me but I can see how they might not be to the sale holder. And that leads to frustration for them because then they don't know why they didn't make much money.
I have a group of friends who loves to go garage sailing (as we call it) and we have joked about making up a checklist of "DOs and DON'Ts" which we could then hand out as we visited sales, with check marks for those things they did properly and those which needed improvement. Just a joke of course! But when I read stories of people who had unsuccessful garage sales, I figure they just didn't know how to do it right. And by the way, sometimes doing it right (in my opinion) means not doing it at all. If you don't have things that will sell well or if you are going to argue with folks who want to bargain*, etc.
*That's not to say you must bargain, just that you need to be polite and reasonable about it, whether you do or not. My son and I witnessed an argument between a customer and seller once where the seller was practically yelling to the customer, "It's a dollar! It's only a dollar!"... repeatedly. We now say that to each other as a joke. It was clear the customer wanted to pay less. It was fine if she didn't want to sell it for less, but she could have just said, "no, sorry, the price is firm" in a normal tone of voice.