When being offered green around here, they call it "purple". I've bought from a guy on the street once, in Tampa. When I told him I was interested, we sat down on bench, and he instructed me to put the money in an empty pack of cigarettes I had, and pass it to him. We worked his slight of hand, and it came back with the purple. If I hadn't had the empty cigarette pack I don't know what would have happened.
I know in the Haight, drug deals take place openly on the sidewalk. Just a straight up barter/exchange. I've seen it happen. Been handed stuff for free sometimes.