Personally, I think the whole concept is bad and what people think of as “admin” is generally wrong. An admin is someone who manages the server and has the ability to modify server settings along with all of the abilities moderators are given. Moderators are there to kick the shit out of bad players. I rarely give my admins more than moderator access + sandbox settings and my moderators hardly get more than kick, mute, ban, etc. That excludes gimmicky commands like rocket and slap. Administration and moderation are jobs, not privileges. Moderation isn’t fun.
Aside from that, I have a rank for my close friends with all of the fun stuff. So, if you’re going to let people pay for extra privileges, at least name the group they get promoted to something like “VIP” and keep it under the admin group so your real admins aren’t powerless against them.
[del]If you’re really set on selling extra privileges, don’t sell anything that would inconvenience other players like guns or commands like /rocket and /slap. Give them access to[/del]
On second thought, don’t sell privileges unless you can offer something truly unique to your players. Selling access to a free addon is just ripping people off.