If you do a search for chandler, most of the results are for suppliers to boats. Dictionaries suggest that one of the meanings of the word is a dealer or trader for a specialized market. Both dictionaries there mention a ship's chandler as one example, without saying that it's the predominant example (the other example given is corn chandler). Both of these dictionaries, and the Online Etymology Dictionary also define a chandler as one who makes or deals in candles. The etymology dictionary gives only this definition.
My guess is that chandler broadened from a dealer in candles to a dealer in any specialised trade and then narrowed to a supplier for boats. Is that right?
Just to confuse matters further, Wikipedia does not mention the any specialised trade definiton, suggesting the word moved directly from describing a candle-maker to describing a ship's supplier.
Best Answer
This is a nice story.
As already noted, the chandler is originally an occupation centred around the production and commerce of candles. The name itself comes from the French chandelier, which in addition to designating the candelabrum itself, used to designate a person whose trade it was to make and sell candles.
Since the fabrication of candles also yields soap as a by-product, most chandlers were also in the business of producing and selling soap.
Ships of course with always a proportion of seamen on the watch were natural consumer of candles and they needed to replenish their stocks every now and then throughout their sailing itinerary. There was indeed no compelling reason to purchase the whole stock of candles needed for a given journey in the home port. On the contrary, well advised shipowners would take advantage of the differences of prices for candles and other products in the home port and in the ports of call in order to level down the total expense.
They would therefore enter in global procurement deals with large volume chandlers located in the various ports of call and open a line of credit with each of them.
They would also have their seamen benefit from the deals not only to increase the purchased volumes and get a better deal but also as a deal sweetener towards the crew - possibly lower wages compensated by additional benefits.
Yet for the seamen that actually was a big advantage because they knew were to get stuff in each port and they would not need to change currencies (at least for these kind of supplies ;-). All they had to do was to make sure they took their allowance of each permitted product.
Besides, even if they'd drink up their wages, the quartermaster would still be assured that they'd have received basic commodities from the chandler's stores.
The structure of the actual financial arrangements was such that chandlers were naturally drawn into extending the range of offered items to whatever ships and individual seamen actually needed, much beyond the initial speciality of candle making.
This is why the profession of chandler gradually shifted from just candle and soap making to that of one stop shops for merchant ships in the 18th and 19th century.
In the wake of previous centuries' English naval supremacy one might say.
Here is an excerpt of the Wikipedia chandlery article: