People that do that are called 'pickers'. I was one for about 2 years, off and on, up in Whatcom County when I was working construction in the mid/late m'70's, had a wife, new born son and a house payment. The economy was down and work was sporadic. So, I did what I could to feed my family and keep a roof over their head.
I still do it now if I see a good deal and if and when I have a garage sale, I keep my prices down to encourage people to do the same thing. It's cash income that doesn't show up on the balance sheet so the feds can take a cut. To me, it's a win/win.