Lorebook: Trade
Since people Somewhere Else do different forms of work or have different circumstances, trade exists as a way that people can use and enjoy the results of circumstances or forms work different than their own.
For example, someone who has skill in fashioning and who has crafted armor may trade that armor for food grown by someone who has skill in cultivation. While some people may do many forms of work, nobody is the best at everything and therefore nearly everyone trades in order to access things they wouldn't be able to access on their own.
As another example, someone who has skill in fashioning but not in general marketing or sales may instead trade their crafts, such as armor, to a merchant, who gives currency in exchange (generally expecting to be able to trade that same armor for a greater amount of currency down the line due to greater marketing connections with or knowledge of customers). That currency can then be used to trade for any sort of good as it is a shared form of payment across many goods.
As yet another example, someone who lives in a region that a certain wild herb not found in most other regions naturally grows would be able to collect and trade that herb for either currency or perhaps a different wild herb found naturally in other regions.
Currency
Currency enables someone with one form of good to more easily trade it for another one independent of whether the holder of the other good is particularly interested in the type of good the trader has to offer or not; it exists to enable people from various jobs/trades to trade using a shared and centralized form of payment independent of that specific form of good and that has a commonly agreed on principal value, generally intrinsic to the currency itself, like in the case of Common Currency. Currency increases a person's options and buying power as they can then obtain forms of goods in trade even if the specific traders they are trading with would never be interested in trading for the type of good that results from the person's work or would only trade based on an evaluation of that good that assigns its value significantly lower than the good's principal value.
Principal Value
Principal value is a range of what a good could generally be expected to be considered to be worth by those who desire or need it. It can be stated in terms of relation to the principal value of another random good (especially one that almost everyone desires or needs) or in terms of a currency.
Merchants
Merchants specialize in marketing and sales as their job/trade and therefore primarily trade the goods that initially come from forms of work other than their own; they are skilled at building connections or finding people who need specific goods thus they usually have higher volume of trade or greater ease at finding people to trade with than people of the other forms of work do.
Negotiation
Negotiation, sometimes called haggling, involves discussing about the value of goods in order to come to an agreement on what will be traded for what and in order to try to get the best deal (either to receive the most for a good or to exchange the least for a good) possible, often the fairest deal in the eyes of the trader initiating the negotiation.