My parents have had a few garage sales where I've also sold stuff. My mom's development holds an annual yard sale weekend that attracts a number of people from the area. I mostly sell old games for $1-10, and one year I think I made about $70.

I don't go out of my way to go to others' sales, though if I'm home at the time of the above annual event, I'll walk around and see what everyone is selling. I haven't found anything particularly cool yet.