The Trade Chat channel will become increasingly chaotic as the game continues to rise in popularity; It only works now due to the lack of participation, either through players feeling intimidated, mailbox-size discrepancies or language barriers.
My idea is to implement a 'fixed-price' Shop for common low-value items, whilst still maintaining the need for Trade Chat (if wanted) and mailbox-size upgrade functionality.
Take Mushroom Paste, for example. It's a commonly traded item, with a very stable price of 40 oj per k. In this new Shop a player can choose to either buy at 50 oj/k or sell at 30 oj/k. The player is paying a premium for the convenience of taking the trade instantly. A better rate (40 oj/k) can still be found in Trade Chat with other players if wanted.
The minimum amount to buy/sell from the Shop is 100 (so 5 oj for 100 Mushroom Paste, for example) and each sale/purchase from the Shop uses the players mailbox size, so a player buying/selling in bulk needs a minimum of 100, but would ideally want more, which keeps the incentive to increase their mailbox further for ease of use when bulk buying.
This idea would fit nicely within the Flea Market, if space was made by removing the value-trap items currently on offer!