Selling Guild Achievements in Cataclysm.
In cataclysm there are new guild achievements which require someone in your guild to acquire certain items be a certain level with a character or kill creature X times.
One thing that Blizzard didn't say was the person had to stay in the guild to keep the guild achievement, so for example you could whisper a GM of a nice guild and say "I will kill 100,000 critters while in your guild for only XXXXg" they may accept so there guild may level to 25 faster. Offers may vary largely because on alliance you can just go to Deeprun Tram and farm those rats, on the horde there really isn't a co-efficient to that. Other ones require NO work at all, (Requires 1 legendary) so find a good guild and say "I have X legendary and I will help your guild get the We Are Legendary] achievement if you give me X gold" or just being lvl 85 and a certain class/race combo can even help. Spam trade selling service such as joining the guild or killing critters or the PvP achievements (Kill 2500 enemy players in their city) this can be more difficult to solo but you can gather a group together and share the profits for a guild that doesn't usually raid cities.
Another way to make gold with this idea requires more players, so you can get a group of friends and sell services such as killing all the classic dungeon bosses, raid bosses, same for the outland dungeons, and raid bosses. (must be 80% of guild).
A third achievement group you can sell is the crafting/gathering ones these can be a much more time consuming achievments but guilds will want their Cauldrons of Battle and Fish Feasts
Not much more I can say, pretty self explanatory. Hope you lucky ducks with legendarys can make some gold and reap the profits