you can keep it free and add something like skins in cs go or league of legends. The best way if a market makes prices on your skins then people will be selling and buying it in many places like mmoauctions.com Then you are an owner of electronic goods
For sure you should take a look on gaming economy like currency. Just take a look of value for buy wow gold mmoauctions.com/world-of-warcraft/gold which is worth more than Venezuela Bolivar right now. Sad but true..