Hey, is it possible that user can buy or sell items with/for money?
or they simply put the currency item in or how?

What I see if that if you set a sell order of the default item (blood), you can gain Economics points by selling blood for another item.  The player is given however many blood you sell in return for the purchase item price.  In other words, you can sell 1 blood for 1 scrap, and the user must have 1 scrap to buy and they get 1 economics point in return.