Each merchants offers a different set of items, but also the same prices of the gear. Rare items found with merchants are very unique so take caution before recycling them, as replacing them can be quite difficult.
As you progress to new areas like Noctis you will find improved and better equipment, but previously visited traders won't have their stocks updated. Some of the traders will also sell blueprints that are required to gain access to new weapon upgrades. New blueprints will be unlocked in subsequent chapters and locations associated with them.