I know this method was posted some time ago here, but just wanted to refresh your memory that it still works, or at least it does now, after green gems prices went to normal.
Fact is, I really don't understand why people sell gems for 7,5 gold on AH when they can vendor them for 9. If any of you has an explanation for this, I'd love to hear it.
Guide goes like this, get Auctionator or any other Auction House related addon (optional), scan for prices, check out green gems and see which ones are under the vendor's price. I know it's not too much, but I've found lots of gems like this and made pretty nice amount of gold from vendoring them. Also, if you find OLD (or new ones soon) gems like Forest Emerald, Monarch Topaz, Twilight Opal, Sky Sapphire, Scarlet Ruby at UNDER 3 gold (uncut vendor price), buy them asap and if you're a JC, you can cut them and sell them for 4,5g / each to vendor or even for 10-15g+ to AH. Good luck