The Trade House unlocks at Level 6 for player to player gold/item trades. Players may start to exchange gems/items at Level 15.
The Trade House is a player to player market system in which players may trade either gold or gems for different items in the game including chest, keys, high quality items etc.
It is against Cloudcade Terms of Service to trade directly for gems/gold. The core gameplay is not designed around the Trade House. This is a player controlled area of the game with minimum restrictions from the development team.