One time, I got 6 techs in a row.
Mining -- BW -- IW -- Metal Casting -- Writing -- Compass.
I sold Metal Casting to Giggly for Meditation, Hunting, Horseback Riding, and Polytheism.
One of the best deals I ever got, especially since the turn after Giggly built the Colossus I declared war with Horse Archers... which he never had.