The characters you as GM play can have the biggest effect on how the players experience your world and story. As the GM, you are the story-teller, and narrating the player's actions as they explore your world is the primary method of exposition the players receive; however, the players will be much more likely to remember a quirky character after a session than the way you described the mountains.
The best thing you can do when plotting out your story characters, is to assign them a core motivation. How does this character live? What do they need to survive or thrive? How do they intend to get it? What is getting in their way? How a character's motivations compare with the party's creates the story for you.
Sometimes, something completely random and fantastical takes the cake. Regardless of your setting, when you need to throw in a character to be an innkeeper, shopkeeper, or quest-giver, consider giving them some random quirk that makes them stick out from the crowd. Especially if you want a character to be remembered, make them easy to remember! Could be an eyepatch, dyed hair, or a cool pet.
The prices listed in the books are an average buying cost to players assuming the item in question is not in particularly high or low demand.
Selling:
Shops will offer 1/4 the buying price as an initial buying offer assuming the item is not in particularly high or low demand.
Bartering
Players can roll Speech vs Commitment to negotiate a better deal. If the players succeed they can achieve a 10% better price, increasing by an additional 10% for every 5 point difference to a maximum of a 50% deal. Critical success will double the price a merchant will purchase at or half the selling price.