I use GTN daily more than any other aspect of the game with volume of my trades reaching 100bn+ per month. Here's my feedback:
The main thing: Add listing multiple stacks in one click, so I could list 20 stacks of 100 particular crafting items in one click, not 20.
Make GTN remember your previous listing price for an item
Add a button to automatically list an item for a market (or -1% of the current market) price, so we don't have to look the price up and set it manually
Add an average recent (7 days) price % column when browsing items, so players would know if the particular item is now listed for 50% of it's usual price (averaged of the last 7 days) or 999%
Add sorting by Cartel items rarity: Bronze, Gold etc.
Add ability to increase selling slots above 100. It is very limiting now and I have to use 7 characters to do all my listings, which is very annoying.
Add current/recent GTN price info in a tooltip of every item in an invenory!
Increase the price limit above 1bn
Add some GTN trading achievements to Legacy
Add sorting by 2 parameters at the same time (name AND price for example)
Probably impossible, but I'll still ask: add orders, so I could place an order for an item if it's not on the market right now and players could see it and fulfil it.
Just sketched these up in 10 minutes, maybe will come up with some more ideas a bit later. Good luck!