Just some more basic info:
You don't get money from killing stuff in stages. You mainly get money from rewards during or at the end of stages. You can pick up what the monsters drop, but generally they sell for very little amounts of money. Occasionally, boxes will drop in a stage after you kill a monster. Loose boxes are the most common, and you can sell them or extract them (don't worry about this yet). Rare boxes and unique boxes are better, and some higher lvl items from these can sell for a lot to other people. To open a box, click it in your inventory, drag it to a key, then press that key.
Also, at the end of stages there will be a reward. If there is a boss, which most stages have, you will get a boss box after killing it. They open the same way. You will mostly get equips from it, although sometimes you might not.
And most importantly, don't buy equips from npc's. You will mostly use equips you found yourself while staging or ones you bought from others. For now, just use the ones you get yourself. The npc equips are very expensive, and are more useful for high level characters.
imrlybored-stage dainn lvl 53