The stw store is where you can spend stw points that you earn from completing quests and other objectives in the game that give rewards.
You can access the "spend stw points" subsection from the options menu. Each item has a detailed description so you know what you are buying.
Please be aware that stw points will not be refunded upon use! If you buy an item that you later wish you could trade for something else, you will not be able to return the item for points.
You can find someone who is willing to trade for your item.