Fixed for the next version, the valuation of trade items wasn't updated after a completed deal while staying in the trade menu.In my game the AI will buy with the same price for a single repeatedly, but will buy for less for each sucessive in a stack. So 4 horse will sell for 2 each if I sell them individually, but in a stack of 4 will only sell for 5. This way its never good to sell more than one at a time.
Also, the valuation of each item of a strategic resource is now always a multiple of 1 GPT, this prevents rounding issues (a combined deal being valued differently than the sum of its parts).