You don’t have to pick a special day on Market Street to be yelled at by strangers. And it’s not that unusual to encounter those in odd outfits trying to sell you objects and services of ostentatious uselessness. But Saturday, the “Cries of San Francisco,” put on by Southern Exposure, offered a witty and sometimes touching variant on an old theme based on The Cries of London, Francis Wheatley’s seminal 18th-century oil paintings depicting London’s street sellers.