This will come with playing, so no rush. On the later levels you'll most likely have to buy health and mana potions before each level. That combined with you selling your items for 500,000 means this is one easy achievement!
To sell an item go to the merchant and talk to him with then press again to sell items. Highlight the items you want to sell, then press to sell it. You can not sell items unless they're unequipped.
To buy an item talk to the merchant with , then press to buy items. Highlight the items you want and press to buy them.