Sell the collectibles
You get the most of your resources by fighting and picking up the silver scattered around the realm. You can make even more money, especially if you have a tendency for collecting stuff. In each region, you will find artifacts that you can later sell. Not to mention that you will receive XP for each collectible you pick up. Completing a full set brings an even bigger bonus, not only in the form of silver, but also large amounts of experience.
Once collected, the items will be permanently saved in your codex, so you can return to collectibles at any time. You don't have to keep it in your inventory. Sell it on the first opportunity! This is one of the best ways to earn money, especially in the early stages of the game.
You may also be tempted to sell redundant equipment. You will be able to buy it back at any time, although it is unlikely going to be necessary. In general, the default gear that Kratos is using from the beginning of the game will not be as powerful as items that you acquire along the way, even if it is upgraded. Hence, it can be a cash reserve in case you can't afford something in the store.