I see how this could work. Maybe a "Personal Add" kind of system where it goes <Player> Seeking to Buy/Sell < (X) NQ/HQ [Item]>.

And then it could allow offers to be posted for the on sale items, requests to be fulfilled in which you say "Willing to pay (x) Gill for [Item(s)]" and the retainer holds the amount of gill in what would amount to a holding account IRL.

If implemented right this would be an excellent idea. And I highly doubt RMTers and Bots could take advantage of it as it'll likely tip off the GM task force. Or worse, allow the Task Force to set up sting operations by posting rare items for stupid amounts of Gil that only a Gil Seller or their customers could afford.