When people sell their old/used/worthless things on their front yard at bargain-basement prices. Called such because people often clear stuff out of their garage, put it on their front yard, and decide that instead of throwing it away, they may as well sell it.
If you add a "b" between the "r" and the second "a" in garage sale, you'll see what a garage sale really is.
n. When people who want to earn an extra buck throw their lameass crap on their front lawn and ask people to buy it for slighty over what they paid it for. These events are usually frequented by old people, soccer moms, and cheapskates.
Garage sales usually smell like old people and antiques. I didn't even know antigues smelled until I went to a garage sale.