With no skill in trade, you purchase commodities for 110% of their base value multiplied by their market supply modifier, and sell commodities for 90% of their base value multiplied by their market supply modifier.
At the start of a new game, the market values of goods across all settlements will need between 1-2 weeks to stabilize. In any given settlement, there will be the base value assigned to specific goods which can be influenced by a buy price (what you pay), a sell price (what they pay), and a market supply modifier (based upon local abundance or scarcity of that type of commodity). As a party skill, if your character has 10 ranks in Trade, you can have a maximum effective skill level of 14, worth a 70% reduction in the trade penalty. Ranks in this skill reduce the difference in price between what you can purchase goods for and what you can sell goods for by 5% per rank.
Trade is a skill available to players and NPCs. The help window that pops up the first time you initiate a trade.
