i'm german so please forgive me my english 'skills' =P
I was worried a lot too which items to sell and which not but you can't do so much wrong and complete the game even if you sell all your items before you get your dragon tower.
you have your own runners who get the materials (herbs,ore,gems) you want, once you own the dragon tower. but you should keep malachite gems if you have some (the malachite ore is more or less useless)
i don't want to spoil anything but you may need a healing potion, an apple, a beer and a rope sometime in the game