I can name about 50 reasons, but I know it'll be a waste of time.
One of the main ones that you can consider is how easy it would be to manipulate. What would you be able to trade? Items? Are you wanting staff to make special exceptions for GST or event items? Seasonal items? Hats? What about GST hats? Tower hats? Seasonal hats? If no to all of them, then's the point? If yes:
- I can manipulate the rewards system in towering by giving many to noob accounts to trade later
- I can turn the GST into a boring joke (more than already)
- I am no longer excited for seasonal events, and the little excitement they provide anyway, because I can just get everything at a later time
And so on, etc.
The only purpose I see a trading system would serve is to give players things they don't deserve. I feel like the integrity of towers, spars, and seasonal stuff should remain limited to the people who were actually a part of it. If not, they end up losing all meaning and it becomes boring as hell.