Most of us don't walk around with magic markers in our pocket.
But sometimes that's too bad. Because every now and then you encounter a "Free" sign next to some curbside items that, in all honesty, can only be categorized as trash.
So there's a temptation to add "OBO" to the sign. The implication, of course, being that the people who put that stuff out really ought to pay someone to haul it away.