Wrong. I recently read on the mojang website that they clearly stated not to charge money for in game currency.
That's because you have a rank. If you die when you don't have a rank on factions you pretty much go survival.
“You cannot make money with Minecraft without our permission… If you are on a server, your experience should be the same as every other player. We just do not want people to mix the pools of ‘paying people’ and ‘nonpaying people’.” “You can charge for hosting servers, but not for gameplay features.” It is seriously not allowed and mojang plans to back this with lawyers. I suggest not taking rank donations unless noobcrew comes into kingship of Sweden. If mojang seriously does back it with lawyers I will lose the last bit of respect I have for them.
Factions is pay to play. Its hardly any fun unless you donate. I guess that's a matter of opinion tho.