There should be a website/app that is specifically focused on just garage sales. Anyone could add photos to a garage sale listing, showing what’s for sale there — this would help out other bargain hunters that are running around to other garage sales.
On the app, you’d have the ability to view what sales are near you on a map.
Text/photo comments of stuff for sale by users if they want to add them.
If owner posts, they could choose to get texts that day with questions on if something is still for sale. Or emails or phone calls.
I’m going through the past few years of ideas that are in my moleskine notebooks. Some of the ideas are really lacking, but might give you an idea? All of these ideas are yours for the taking. Don’t forget my archive of 100+ web startup ideas.