Is it permissible for a volunteer zakat collector to take a share from it, or is that contingent upon him being appointed by the ruler?

The questioner is not permitted to take from zakat in his capacity as an administrator over it, because he was not appointed by the ruler, and he is considered an agent for the zakat payers in its distribution. However, he is permitted to take from it if he is one of those eligible for zakat, such as the poor and the needy, provided that the zakat payer has not designated it for other poor or needy individuals.