Currency made by devs or auction house ran by npc/game server itself could be interesting but I don't think that it fits the theme. Things should be player and community generated.
If something, running a market via barterstand and coins should be made easier for both keeper of the market and user of the stand.
Possibly going as far as giving barter hands unlimited range so that you'd not need to risk giving your items to the keeper of the market.
Yes it would be rough and give possibility to insta tp items for other purposes as well, but it would make trading more easy.
Simply increasing capacities of barter stands and removing discovery requirements for items on offer would be nice too.
Adding auction stands could work too. For example:
Auctions are started with duration and minimum price, buyout price, people can offer more pieces of the item requested to become the leading bidder (basically coins would be mostly used as items for bidding). Both the bid and item being sold would be stored in the barter stand itself. When you place a bid, you get a ticket that you can use to either redeem your bought item if you won, or your payment if somebody outbid you. Ticket could be used remotely even, or possibly via the stand as well.
Only way to get items out of barter stand via other means would be to destroy the stand. So risk of seller/buyer cancelling the transaction or something silly is small, as it would be noticed easily and could be somewhat controlled in community market environment via trust.