Commercial, mercantile are comparable when they mean of, relating to, or dealing with the supplying of commodities.
Commercial is the more widely applicable term: it may be used in reference to anything which has to do with the buying or selling of commodities for profit, with their transportation, and sometimes, aven, with their production, or with business affairs in general; thus, a commercial transaction is any piece of business involving a buyer and seller of goods or property that is for the financial benefit of the seller; commercial law deals with all matters (as contracts, negotiable papers, liens, payment of debts, and partnerships) that have reference to business; a commercial attaché is, in the United States, an officer of the Department of Commerce attached by the Department of State to an embassy or legation in a country where trade is important.
Also, commercial is used to describe whatever has for its aim financial profit or is guided by the methods or practices of business.
Mercantile is often used interchangeably with commercial with little difference in meaning; thus, a mercantile transaction is not ordinarily distinguishable from a commercial transaction, nor mercantile law from commercial law. The term, however, more often suggests actual buying and selling (the occupation of a merchant) than commerce in general including production and transportation and is therefore more restricted in its application; thus, a commercial house is a business or company engaged in foreign or domestic commerce; a mercantile house is a business, usually wholesale, engaged in merchandising.