You may be taking the word “merchant” too literally, especially against the background of a more current day use/interpretation.
Not sure about Scotland but in England a coal merchant was also another name for a Coal Man (Coalman) - a person who sold coal (usually back then from a horse and cart) house to house. In latter, e.g., as “recent” as the 1960’s a coal man still delivered coal to people but by lorry (truck). However the delivery person was then typically an employee of a local distributor from which one ordered coal to be delivered as required or . . . as in the winter months . . . had a standing order to deliver x-amount of coal at regular intervals. The house I grew up in London was typical of a Victorian structure that had a cellar in which coal was stored. The coal was delivered via a manhole in the front path of the house that allowed the coal man to empty the sacks of coal down it.