Hi,
right now, traders ask you sometimes for stupid tradeoffs, like “Give me two wooden swords and I give you one wooden lantern” or something like that.
And maybe you do the same as I and click “NOOOO WAY” 
But what if traders would have personalities which react to your answers.
What do I mean?
Let’s do it with an example:
Trader A is a good guy but does not really know the worth of stuff
Trader B is a not so good guy but knows exactly the worth of stuff
A wants two farmer’s hoe and gives you 1 stone garden lantern for it.
This is not a good trade and you say no
B does the same thing and you say no too.
Since A is a good guy he is not affected by the NO but B is a not so good guy and dislikes you a little bit now, which he tells you.
A comes back and wants a wooden sword and gives you a stone maul and you say yes because it is a good trade.
B comes back and wants a stone bench and gives you a bird bath but you do not want that trade and you say no.
Trader A likes you more but trader B dislikes you now.
Trader A will come more often and gives you better trade offs but trade B will come less or even not at all and gives you even worse trade offs.
In short, if you say yes or no to a trade the trader will change his/hers attitude to you.
This will effect every next trade to a point till the trader hates you or loves you.
And like in my example traders could have different personallities which add a deeper layer to the trading system.
A good trader will not be affected by a NO, a bad trader could not be affected by a YES.
A trader who knows the worth of items will give you different (good for you or bad for you) trade off if he/she likes you or not.