Design Observer has a slideshow of Michael Zinman's unusual collection: signs he has purchased from panhandlers.
I did engage with all the individuals I purchased signs from, and quite often, my offer of purchase was declined. I would guess at least two out of every five people on the street turned me down, and I was not able to purchase their signs. They were just unwilling to part with them. I think it was a matter of self dignity, and I was ever sensitive to their condition and never tried to further persuade them to sell.