Cataclysm greenies are still selling surpisingly well, so everytime you disenchant or vendor a level <-80 greenie, Deathwing kills a squirrel. A level 77 greenie with lots of useful different stats may sell for even 400g! Though personally I list all level 77-80 greenies for 250g ish, sells faster too. And theres new alts looking for "epic" gear every day!Do you want more gold tips? Don't forget to share!
If you want to farm these valuable items and you're skilled, level capped pet class or a tank, your best bet would be to solo farm Cata starter instances, like Blackrock Caverns. Elites have a higher chance of dropping useful equipment. However, there is no real level requirement for this method, as long as you can kill some Cata monsters.
All monsters have a low chance of dropping greenies, so it's up to you to choose where to farm. The lower level the zone, the lower level items you will find there. So, I'd suggest either of the Cataclysm starting zones.
If you have any level 80-84 toons, it's a good way to combine farming gold and leveling by grinding, or just do your regular questing but do NOT disenchant the loot. Even "of the whale" will get you at least 100g, if you're patient!
Personally I farm in Deepholm. The mobs are a bit higher level and will often drop level 80-> items which sell for a bit less, but the occasional level <- 80 items are pure gold.
Don't forget to drink a Potion of Treasure Finding before killing any monsters!
Alternatively, you can check AH if people are selling these greenies for less than 100g. Usually people don't know how valuable they are and list them for less than 40g. Since when do epic quality items go for that low prices? :o