For those players who simply wish to obtain the spoils of war without going to war, KingsRoad does allow for the purchase of the various pieces of Item Sets directly from the Shop, for both Gems and Dragon Stones. But you cannot just outright purchase the highest qualities of Item Sets available in the game.
The Shop will show Item Set offers based on what the player has already achieved on the class they are playing at the time. If you've only managed to get through the Bronze difficulty of an Event and only have a piece of the Legendary quality Item Set equipped, then your offers will be limited to the same Item Set. But if you've managed to get through the higher difficulties, then your offers will be of higher quality in turn.
Please keep in mind, as noted above, that this is based on the current class you are playing. If you want to kit out your Archer with the best gear, you'll have to have at least gotten a piece of that high quality Item Set on the Archer already.