Mana is generated by tokens that are "static / not moved". Once you move these tokens, you can assign them to an identity.

Instead of assigning the mana that is generated by a transaction to the owner of the tokens, they could also be assigned to somebody else.

This moment where "generated mana" is turned into "accounted / assigned mana" is the only time you can "sell" it as afterwards it turns into an identity bound token.