Blizzard posted the official statement concerning gold selling and power leveling services. You must read the full article here.
“Players who purchase gold or use power-leveling services are not always aware of how these actions impact the game and our development and support efforts. With that in mind, we’ve posted a statement that sheds some additional light on these topics, including information on how purchased gold is obtained and on the potential negative consequences of sharing account details. We hope anyone thinking about buying gold or sharing account access for any purpose will read and consider this statement before taking actions that could compromise their account security and result in other negative experiences for themselves and other players.”