We do not normally allow people to sell their in-game items for real world currencies (because it competes with our own revenues) but with a jewel system you would simply sell your in-game items for packs of jewels which you could then sell on any exchange to cash out to fiat currency.
So basically you will now be allowing in-game items to be sold for real world currencies using Jewels as an intermediary step.
How is that not competing with you own revenues?
Because of this, the game will attract a much larger sub-audience of those who are trying to farm in-game items for cash. While at first glance this may sound like a bad thing, it's really not.
How do you avoid botting? There are already bots for Conquer Online. How do you prevent users adapting them to farm items and in-game currency on Apex, making the price of in-game item drop and sparking massive inflation of the in-game currency?
This creates a higher demand for the jewels because not only can they be used on non pay2win elements (cosmetics) but now they can be used to purchase actual in game items and bonuses but it is done as peer2peer and therefor does not reflect poorly on the servers balance.
If you allow players to sell items that give an advantage in-game, like weapons, you are basically allowing Pay2Win. It's not Pay2Win between the players and you, but Pay2Win between players. A new player can basically come and buy $100 worth of Jewels, buy XP boosts and equip himself with topnotch equipment, and outperform players who have been playing for a while. I'm not saying that's necessarily a problem, Diablo 3 has been doing that for a while with the Real Money Auction House, but let's call a spade a spade.