An iron merchant (though an unusal term) is one who sells the metal iron in various forms (look up steel stockist in your telephone directory)
An ironmonger, which derives from the words iron seller, is one who sells nails, screws, tools, cutlery etc ie goods originally made from iron/steel - in American possibly a hardware store though I would recommend the use of a British english dictionary after all the word has been in use in english since about 1350 !