The idea is pretty simple. You are trying to get the best deal for an item of your choosing. Whether it's a graphic tee or an old action figure, whoever can haggle for the biggest discount is the winner.
Take it a step further and whoever can find the best value, meaning what do you think the owner paid vs. what price did you pay at the garage sale for it?
Final game variation, whoever can get the MOST items for their money is the winner.

Usually, garage sales are linked to "nice weather". No one wants to be walking around in the rain from house to house in your ritsy neighborhood, trying to find bargains.
Most often, you can find listings for garage sales in the newspaper, Craigslist, or on Facebook. I remember when I was a child, we would have a neighborhood garage sale. All the houses around the neighborhood would collectively put up signs around the area and, on the agreed upon weekend, would open their garage doors to any roving weirdos who wanted to see their old stuff.
tl;dr check the newspaper, craigslist, or facebook.

Often times, homeowners will group together and host a neightborhood garage sale. You can pretty much bank on parking your car somewhere in the neighborhood and then walking from garage to garage as you play your games.