About the idea to trade good for other goods, I don't think that can ever work, it'd overy complicate the whole idea and is best left to direct trades between players.

The buy order idea though, that's decent. In most cases the limit would be the preservation of the money unit, gil in this case. If I put up buy order, the game would have to set some of my money aside so the buyer can be sure to get their pay without me spending it in the middle. However this game offers a good ground for that in the form of the retainers handling the sales, in which case to put up a buy order the retainer needs to have the required money on their pocket.