A Sell Box
Just like the buy box you set a number, and type the name of the item you want to buy using the Sell Box. Then someone can sell you the item for the desired price. The money comes from the player who places the box's balance. Or, you could place a safe next to the Sell Box and the funds would be supported by the safe. Only one item could be sold so that people cannot steal all of the buyers money.
This is an okay idea but, the trading system is already fine in my opinion, it's not like back in 2012 or so when people could easily scam you.