I am... well, just call me the horned statue. I was cursed to take this inanimate form by the Goddess for tampering with the essence of others for profit. It's simple. I'll give you 100 rupees to take one of your Heart Containers or a Stamina Vessel off your hands. You can then choose either type of essence to buy back for 120 rupees. Exchanging a Heart Container for a Stamina Vessel, or vice versa, for only 20 rupees sounds good, doesn't it?