So with that in mind lets move to what the current problem is with trading.
if you visit the trade worlds (mainly w1) you'll notice that almost everyone in that world is standing in and around the west bank in Varrock. This means that nearly 2000 people are standing on top of each other and trying to talk at the same time. This leads to peoples chat instantly being removed from the chat channel, making it hard for some people to trade, some people also use Auto Typers to spam the chat trying to get their message across (even though this is a mutable offence for macroing & spamming). Even though Zybez is used in place to make trading easier.
trading board and falador trading are the best compromises...
trading board you can type a message (leave an ad) up (up to 6 ads) and walk away, do what ever and when someone sees it they can pm you, and you two can teleport to the sellers location across world depending on who is in the safest area + least populated world...
So the complex part...chalk boards can be placed in every bank in the game (convenience), you only need to type once and it doesn't have to be in the game chat (no spam), it's easier to find people (buyer-seller cross world teleport), a search feature can be implemented so you can find what you are looking for and the price you want to pay (item/gold value required to pm person to avoid scams), since you have to be online to trade the Ad will only be available while online (no fake ads), there would also be a limit to how much of anything you can place prices would be set by the player and not based on market flow.
maybe introduce a trade limit for the first 24 hours on a newly created account or new accounts are unable to (drop) trade for the first 24 hours of consecutive gameplay, that would also stall gold farmers/bots a bit.
but remember scamming can be prevented, bots and merchers are always going to be an issue.
G.E. people: more time playing and less time waiting,
Both: very convenient, easy to find people, no spam, and scamming prevented, price manipulation prevented
Anti G.E. people: no offline trading, player to player trading, less dev time required, prices player controlled
the trade board can also double up for other things, it can be used for events, clain recruitment, looking for someone to quest with ect.