Another easy, useful idea, for someone (like me) who might accidentally buy more than they should have, and (like me) then find they cant pay upkeep because they don't have enough gold, or time left (to make gold trading), or turns left (to work as porter). (Yes, I got myself into a pickle thinking I was being super efficient, but forgot about upkeep).
Anyway, like I said, easy and useful idea: Sell back the same trade item as is bought in a port, i.e.: Buy too much cotton in Caspian, realise you've made a mistake, then sell back that cotton in Caspian, breaking even and effectively undoing your mistake.
Side effect of idea: it would be possible to buy an item one day, and sell it back the next in the same port and make a profit due to price fluctuation, but the profit would be much smaller than that possible shipping the item to another port (only 2 gold per item, at most). I don't think this effect would be a major game changer, and probably would reflect a type of trading that regularly takes place anywhere at anytime realistically (trading on fluctuations within a market rather than differences between markets).