Trading should not be for coins but for other good simple as that.
Each object should have a “value” stat depending on the quality and material of the objects, for instance a normal Oak chair would have a value of 40 as one of the most common materials, but a better quality wood for instance Mahogany or similar would increase the value for instance as it follows
Chair
Type Material - Base value - Quality
Pine ____ 10 _______ 10 _______ C 20
Oak ____ 13 _________________ B 30
Cherry __ 16 _________________ A 40
Walnut __ 20 _________________ S 50
Birch ____25
Mahogany 30
So whenever you get a better quality item it would come to the base value of each item and be modified depending on the resulting quality of the crafting and the material used.
so it wouldnt need to create a table for each item but for each type of crafting
a.k.a Metal crafting / Wood crafting / Cloth crafting / Leather crafting
so you could trade the items accordingly
the better relation you have with the trader group
Lowering the trader requirement for a better offer.
usually with a neutral level trader you would have to have a better offer
so when trading with neutral you will have to give a higher value items than the items you are asking for and that would be lowered depending on the charisma of the trader you use and the better relation you have with the trader’s group.
lets say
you offer
2 Oak chars
2 Birch tables
1 gold candelabra
1 Bear leather armor
total value of 460gp
you could ask for anything below 400gp
anything above that would get a negative answer from the trader or make him angry making him leave or his group declare war on you.