I have actually been on the other side; about a year ago I needed some quick cash so I filled a shoebox with some of my lower end, identified Roman coins and actually sold a handful of them; about a quarter of what we made at the whole yard sale. There was a lot of interest - much more than I thought there would be.
I probably wouldn't attempt to sell any coin worth more than $5 at a yard sale though... too easy to get your buddy to distract the seller while you slip some in your pocket! Or worse yet, get your house tagged, and come home from work to find your house broken into!