A major portion of the game is economics.
Base prices of items change based upon difficulty so when in the store mouse over them to determine how good of a deal you’re getting.
After looting a battlefield don’t just sell everything, repair equipment and then sell it. Use the money to purchase more tools. You’ll wind up with a larger profit than if you just sold the damaged items. However, a few items are not worth the effort. Clubs, knives, daggers and any armor with a lower rating than 55 should be sold outright.
Early game, don’t sell the daggers or scramasax you come across. As a secondary weapon they cost 0 or 2 fatigue and your troops can overwhelm a heavily armored foe to land their special Punch attacks, bypassing the armor and allowing you to loot it.