Getting reach and trading are important parts of Pathway. Thanks to money, you can buy new items for your expeditions (e.g. medkits, weapons, armor, ammo or fuel). Ammo and fuel refills automatically to the default level every time you start a new mission. However, these supplies aren't enough to complete a mission nor reach a destination point.
To sell or buy items, look for a pouch icon on the world map. Visit that place to meet a trader. You can return to him whenever you want, but his offer will not change on subsequent visits.
You can sell every item in your inventory - click RMB on an item and select "Move to valuables" option. That item is moved to the valuables tab - it won't take up space in the inventory. You won't get the money right away - the content of the bag with valuables must be sold to a trader. Another way of selling valuables is to complete a mission successfully.