I've made a countless amount of money selling guild tags to other players on RP servers in the past. I've done it on my alts and on multiple servers from 2006 onward. One of my guilds sold for 1000g, with the lowest sale price being 200g. Players on RP servers want to look like they're a part of NPC factions. Since Blizzard only pre-reserves some of the factions' names for the guilds, it's a good idea to purchase up a bunch of guild charters and create some guilds. Auction them off on trade chat when you start to see a lot of players on RP servers following a trend.
Here's some hot sellers for me in Cataclysm:
<Gilneas City>
<The Highborne>
<Highborne>
<Citizen of Gilneas>
<Twilights Hammer> <- (Make a GM ticket to change this to <Twilight's Hammer> for proper grammar. They'll do it)
<Twilights Hammer Cult>
<Guardians of Hyjal>
<The Consortium>
<The Stormwind Guard>
<The Stormwind Army>
<Elwynn Rangers>
<The Elwynn Rangers> (Make grammatical variations and sell them off.)
<The Dark Rangers>
<The Bloodpack>
<The Kor'kron>
<The Warsong>
<Warsong Offensive>
<Noble of Stormwind>
<Noble>
<Noble of Gilneas>
<The Deathstalkers>
<The Scarlet Crusade>
<Scarlet Crusade>
<Scarlet Onslaught>
<The Argent Dawn>
<The Wildhammer Clan>
<Wildhammer>
Auction these off in tradechat. Ask people to PST you for offers. Anything to do with the new race/class combos will make roleplayers go wild; trust me, because I am one. Profit off of others!