The "true price" is a base price of 100 units of resource (for the case of some resources being of more worth than others). With current bartering system only relative values makes sense so if for example stone is two times more expensive than wood, there will 2:1 exchange rate. Constant part of price percentage difference between true price and buy/sell price. Buy price equal to true price plus constant difference. Sell price equal to true price minus constant difference.
Additional difference of prices in percents, added after each deal to specified resource price.
Price difference percentage which restored each restore timer tick
Interval of timer which slowly restore prices after deals