Can I sell Energy on eBay or Gaming Open Market?
No. Energy is intended as an in-game credit to be used within the game to purchase goods and enable aspects of game play. It is not to be resold or traded for anything else outside Spiral Knights. We will ban players who attempt to execute such trades.
(taken from spiral knights billing faq)
This means you cannot try to sell CE on Ebay or in any other way convert it into $$, receive $$ for it ect.
You can only use Steam Trade functionality to get Steam Games for CE, or other available items.
Trying to make a direct trade of CE - $$ will result in player ban.
Hopefully it explains the issue O_^