The release notes to update 19 include this "Each roll of the Daily Dice system grants a prize of Experience points in addition to a prize. These points range in amounts based on how well you roll. The amounts bestowed by the Gold roll are higher than the Silver roll. This XP is granted as an item in your inventory that is bound to account. To activate it simply click on the dodecahedron in your inventory and it will bestow the XP to that character."
You cannot sell XP like this. It subverts the entire system.