According to Wikipedia, this term was first used in the 19th century by Bible salesmen, working for the British and Foreign Bible Society in the French Pyrenees. It was derived by merging the French words col (meaning the neck) and comporter, 'to peddle'. Porter being the French word for 'to carry' (from the Latin portare), the implied meaning is 'to carry on one's neck'.
Wikipedia (and other sources) point out that although the English words 'colportage' and 'colporteur' originally meant the selling of Bibles and other religious tracts, they later came to be used to refer to bookselling in general.
© Haydn Thompson 2021