Looking up Larousse's Grande Encyclopédie I found no entry for marcelot, but for mercelot: a pedlar in small books in the 18th century (article: colportage).
In our French-German dictionaries it is mercerot (petit mercier - you guessed right, Malcolm): a pedlar/peddler in haberdashery = a wandering dealer in yard/piece goods. This would suit the scissors in the song very well (brit./am., the dict. gives different words for both sides of the pond).