I have often had trouble dealing with trade because I wanted a high public price so as to tempt the random traders, but while wanting to maintain the ability to buy food cheaper. I have had many lords saying "I would be more than happy to sell food to you cheaper, will you lower the price?". However, I can't, as it could screw other inbound caravans and make them turn back.
As it is, caravans going to sell food have a minimum sell price, but not a maximum sell price, which really complicates and limits things when it comes to treaties and agreements that concern food. I would be really nice if lords would have a checkbox when sending out food so that it sells at the specified price, even if the region would be willing to pay more.
'cause as it is, I'm paying food from some people at a higher price than both of us are willing to agree for. This truly limits the potential of having treaties with some realm where one will offer cheap food in return for something else (if anything). The alternative is that I send caravans to their regions, but then the risk is that someone else picks up that food before me, as they can't reserve that food for my caravans.