Even if trading was added it would most likely be limited to only allowing players to trade public items that would have been purchased from shops.

Items like towering and GST hats would probably be something bound-to-account and the same goes with VIP items and treasure hunter rewards because those all things you should be earning your self if you really want it, well you don't really do much to earn a VIP item but if a player misses a month they shouldn't have the item for it or else what's the point of it being VIP?

Real-world trading would become a reality and that's not something Graal would want to support for obvious reasons.

Lastly, developing a trading system is kind of a waste of time since it serves no real function and they can focus their efforts else where.

The only pro here is pretty much being able to obtain old items you couldn't or didn't buy.