As you work your way through the game, you will come across numerous Shops - NPCs who sell Quest Items of varying degrees of importance.
Tutorial Shop (Haven Castle)Edit
Where you are introduced to how buying from shops work.
It has only a single item for sale, the Rusty Sword, which is purchased as a part of the Tutorial.
The biggest and most important of the shops in the game, within are two NPC shopkeepers - with a third, Diane the Magnificent, appearing upon reaching Level 30.
This is the place you'll go to purchase the majority of the gate items in the game, as well as numerous other items that, while not necessary to the completion of the game, will make things easier if they are purchased.
There is also another NPC present who will explain Bush Bucks to you, but sells nothing.
Only accessible after completing the Level 30 Main Quest Set, as well as completing the Side Quests issued from Mr. Smith himself for building a smithy for him, it's an excellent source of both Daily Quests as well as unique Weapons.
Upon reaching Level 60 and heading through the northern gate in The Frosty Canyon, you head west to find this shop in the The Haunted Village.
Upon reaching Level 80 and opening up the Great Bushier Reef area (the northern tip of the Tropical Islands), the shop becomes instantly available.