Why is it called a Flea Market ?

This is a jocular term for an open-air or street market for mainly secondhand merchandise, which would be the type of items that might be infested with fleas. The first flea markets were in Paris and they were called marché aux puces which translates to 'market with fleas'. Flea market first appeared in English in 1922 as a translation of the French market's name. Flea is of Germanic origin (fleah) and was not spelled flea until after 1550. A synonym is flea fair