No one is going to send a message halfway across the skies to aquire a resource that could have been found in their own port at half the price, especially if that resouce is a regular product of that port.
In the case of a bargin said cargo can then be bought and delivered at the same exact bazaar, no one is going to miss that.
The game makes some notion towards the idea that this is a contract by saying no other captain will take on a prospect you have claimed, but then you can also underdeliver on or discard a prospect at will, flying in the face of any suggestion that this is contractual in any way.
This is without even considering how mechanically dissatisfying it is.
It’s also a fairly simple fix, exculde some ports as the destinations for particular cargo, and prevent ports from having bargins on cargo they are currently requesting