Quick win here: please
limit the buy or sell quantity from a town, to correspond to the number of vans at the town. This prevents unfulfillable orders from being generated.
Here's why: I see offers (6 just today!) of more than 210,000 of something, from a town. It's impossible for anyone to fulfil these orders.
For example, someone wanting to sell to meet your buying order for 300,000 goods can't muster enough vans (100) to meet the (only) 'Sell All' option that they have.
It's likely that the players making these orders forgot to select their Trade Hub, or were allowed to enter an impossible quantity. They're likely to be unaware of the error, until someone (like me) taps them gently on the shoulder to explain what might have happened.
A validation of town-based orders would prevent unfulfillable orders from being generated.