depending on the region you're at, go to a village and ask villagers as well for a quick idea.
But usually, at least on some items:
Senderfell- Buy all iron under 200
Pointsbruck?- Buy furs under 200
Javiskholm- Flax (try to keep it under 100), also villages usually have A LOT as well and you can buy in bulk and they stay at the same price.
- Flax seems to sell well only into avendor however.
Laria- Oil, not sure price range, but under 300-400, but keep it @4-6 units, not worth after that.
Singal & villages- DATE FRUIT UNDER 70, usually 40-50, holy crap this is the BEST thing ever, when you go into d'shar lands, stock on these as they sell for over 160 on almost every other town.
Then you have velvet&dyes, but honestly these are only worth buying in low quantity as the return is quite low...
Buy salt from Nal Tar or Torbah and sell it into any other town other than d'shar, these also sell well, buying from as low as 120 and selling up to 370.
Do remember you need a good trading skill for most of these to be worth it, if not, stick to date fruit, flax and salt routes, the rest give little return.
AGAIN: DATE FRUIT SO FREAKING GOOD TO TRADE EARLY ON, almost all d'shar villages have crapton of it so buy for 50-60 max, I don't even bother buying from towns since they increase in price quite a lot. And make a killing of a profit selling it slowly to other towns (don't dump it all into 1 town).