Special items are available for Obsidian Crowns ingame from Obsidian Crown merchants. Players sell sometimes homes, lots or deeds for crowns too. The Crowns you can buy from other players for ingame money, buy from the website for Dollar or have some earned from Kickstarter pledges or ingame quests.
Player can have private merchants which sell their goods and can have buy orders on them as well. If you open a player merchant window the small number next to your "Sell" tab in it will show how many items they have placed as buy orders. The Outlander Welcome Center player town next to Soltown buys raw resources for example.
Public merchants exist in all public towns and allow player to sell to other player and buy from them.
NPC buy based on demand and offer, so prices change. Furthermore NPC of different professions can buy and sell for different prices - it pays to shop around.