I would be very leary about purchasing fleas from a stranger in some unregualted open air bazaar. Many species can carry Bubonic Plague as well as various malaria type diseases. And if you are buying by the pound I would be very suspicous of any type of scale being used as these venues are rife with thieves and hucksters. Ask yourself, why are these fleas being sold for such a discounted price? What could be wrong with them?
Fleas can be purchased for scientific purchases from various scientific research companies who must meet stringent government standards as to the quality and health of fleas.
If you are at such an open air market and someone does approach you with a proposition to sell you fleas, just say no, turn around and walk away.