The Dealer is an NPC that appears in the game from time to time in order to exchange items with a player. Usually when the Dealer is nearby, when opening the common map there is a notification about that. In order to occasionally receive such notifications one has to make a radio. However, don’t attack the Dealer, this won’t do any good. 
You can use CB Radio to contact the dealer. However, in the age of post-apocalypse radio communication is not as reliable as before and it is possible that you will not be able to learn exactly what items he is willing to trade for.