They would sell things in the shop like keys to open chests you'd find, temporary buffs like exp gain, pets/minions, holiday vanity gear, etc. All of which can be bought with gold after currency exchange. Funny story but I actually played that market for a few days like the stock market back when the game launched. Ended up making quite a bit of gold, like 50-100. Was probably the most fun I had with the game at 80.