Welcome to my modest shop. How can I serve you? I can offer you some good weapons and items which may improve your abilities! Please make your choice... 
The Merchant is where you can buy a vast variety of permanent and unlimited items as well as resell your unused items.
Buying[]
Most items can be bought with just gold coins but some extraordinary one require Hellstones. A certain level must also be reached. If you have reached the required level and have the necessary number of coins (and Hellstones), simply click 
Reselling[]
If you have an item in your inventory, whether equipped or not, you can resell it to the Merchant at any time by clicking 
. But you can only get back, though instantly, 25% coins of the original price (and no Hellstones as for special items).