Singer meets a pretty walnut vendor. He tells her he's sick of single life, and "O won't we be happy until wedlock breaks us here?" Chorus: "Ten-a-penny walnuts, my Nellie she were by/Fresh from Common Garden, please to come and try...."
According to MacColl & Seeger, "When John Lydgate (1370?-1450?) wrote 'The London Lykpeny,' he started a fashion for poems and songs about London's street-vendors and their cries which was to last for more than four hundred years...lavender, oysters, water-cresses, pretty flowers, codlings, cockles and mussels, and even cat-meat." Haven't heard that last one.- PJS
Of course, many of these songs are on rather less pleasant topics. Consider "The Oyster Girl" or "Queer Bungo Rye."
I think the claim a little strong anyway; while Lydgate was a prolific author, just try to find anything he wrote! He's usually lumped with the "other poets" of the Chaucerian era. His may well be just another example of an author taking advantage of a popular street form. - RBW