Trading in COE uses two different game currencies, rubies and ohais.
Note: As ohais' prices have been massively rised by the game developer we would like to advise you to refrain from paying cash for ohais at the moment. It simply doesn't seem reasonable to pay 65$ for a pixel dress or a large bag.
• Rubies are the main currency in game, and this does not involve any real cash payments. The game is fully functional using only rubies for trades.
• Ohais get let you buy special and rare items from merchants, but you will have to pay real cash first to obtain ohais. Curretly this is not recommended, see note above.