The Marketplace is a feature accessible directly from the home screen of the game, where players can spend Gold or Gems to purchase items.
The Marketplace (as of Version 1.28.0) has 29 items total. It holds the same 25 items that be bought once per day. With the 4 available (30x 3★ Specific Hero Shard) randomly changing upon refresh. Every 24 hours every item in the Marketplace refreshes.
Items sold in the Marketplace sell a fixed amount at the fixed price every day. It sells many of the consumables needed to upgrade heroes, and also summoning scrolls to be used in the Summon Circle.
Try to keep 12k+ Gems at all times for Gem Box Events/Grey Dwarfs' Blessing Events (Blacksmith). Try to buy everything listed unless stated otherwise. If it's not on the list then avoid it, it's bad value.