In semi-historical setting (antiquity/early/late medieval). Mysterious merchants arrive, on huge steel ships (or even some flying vessels, like zeppelins or UFO-like spaceships). They look perfectly human, although, nothing like the locals. They are neutral-friendly, but don't let anyone try to bully or rob them.
They offer an astonishing variety of goods, from grain, veggies and meat (in bulk) to 'magic' stuff (future tech). Or medical, engineering, scientific services.
The catch is, they take only one kind of currency: a special amulet has to be worn(a camera), and the person has to spend days (hours) in the nude in public. Women (especially beautiful, especially of higher social status) nude-hours cost a lot more.
