System buyers have a price ceiling
Cities, ports, airports and national carriers no longer pay any price a sole seller asks. Their standing bids also follow the market, and lapse once the buyer no longer needs the resource.
Two refinements to the standing bids a system buyer leaves when nothing is for sale.
A ceiling on what the game will pay
Every system buyer — a city laying out housing, a municipal port or airport refuelling, a national carrier, a public-works yard — now refuses to pay more than a fixed multiple of what the resource costs to produce. That multiple comes from the recipe and its crew's wages, so neither listing nor trading can move it.
In practice this only bites if you price far above the market. A sole seller can no longer set the benchmark for a resource just by resting an expensive ask on the book, and a city will not buy housing materials at an arbitrary price.
Bids that follow the market
A standing bid now lasts two game days. While the buyer still needs the resource the bid stays where it is and its lifetime is extended; when the market price moves, the bid is replaced at the new price. If the buyer gets the resource another way the bid lapses and its money returns.
A bid is never raised just because nobody sold into it. What a resource is worth comes from actual trades, not from how long an order has gone unfilled.