Yea there is some small math behind it, but I wanted to keep it general, obviously never trust the "assess item price" as those are only for 1 item, and as you pointed out they increase in price the more you buy and likewise decrease the profit the more you sell.
But the DATE fruit is just so damn cheap, last time i found it at 23/unit in Singal and villages was 33 IIRC and villages DON'T increase when you buy more than 1, and these are safe bet for buying in large quantity as ALL other towns offer max price for it, which is 163 IIRC, always above 150, so there is always profit.
And its a good way to keep you moving, maybe finding some more parties to kill along the way when you sell your date fruits. I can't believe how profitable this actually is, the rest of the trade routes are like a "bonus" in comparisson.
Date fruit is 100% the MAIN trade route for me, and I usually only bother with flax and iron from there on, sometimes furs and OIL.
Salt also works nice but you can only truly buy it from towns and they can increase in price quite a bit... Sometimes you can get lucky however and buy it from villages.
either way, Date fruit is king, along with his "prince" Salt, the rest are ..."side deals" xD.