Tbh Flo, I know most people are tethered to summer holidays because of children but it's probably the worst type of holiday in my book. Covid has probably made it worse since it's the only 'safe' time to travel these past few years so there's even more pressure on these few weeks.
You are such a veteran of secondhand shopping, I'd never think to pay attention to something like a yard sale. We have the King's day market in NL which is supposed to foster entrepreneurial spirit in children, so it's basically haggling children down to pitiful amounts of money for shit their parents no longer want. It's always way too busy for me to venture into and I seriously doubt any fantastic finds are out there, but there probably are, I still lack the motivation to suffer the crowds though.
In France the have the Vide Grenier, so old stuff from the attics, you can pick up some lovely old chemises and beautiful monogrammed linen sheets and tablecloths but they're almost never on when I'm visiting Paris. I always think to go to Les Puces as they've got a good couple of streets worth of vintage fashion but it's a schlep to go there so inevitably it always gets scrapped off the itinerary.