Here's what should happen:
When buying, attempt to purchase the cheapest food possible. Keep buying as long as there is food for sale under the threshold amounts (gold and food) and gold available.
When selling, sell to the highest offer available. Keep selling as long as you have food for sale, there is an offer to buy it, and that offer meets or exceeds the price you set.
Buy low, sell high. Auto-Caravans should employ merchants who understand that. After all, the concept of "auto" or "manual" isn't something that they should be aware of.
