After the announcement of Expansion Reaper of Souls, Diablo 3 Gold prices has increased 0.6 USD per 100 million. A month has passed, the gold price has been normal again. The gem in the auction house has seen as big fluctuation as well. There would be a bigger price fluctuation when the Loot 2.0 comes out and Expansion come online.
Loot 2.0 release date analysis: There should be something big to be announced in December Blizzcon. There is a big chance to announce Loot 2.0 instead of expansion on that day. Maybe one week later there would be Loot 2.0 and one or two month later, there would comes the ROS.
Should You buy gold or sell items right now? A lot of players suggest that Gem should be the safe investment not the gold. If you would like to make a small fortune before the expansion, you should buy gold and buy cheap items before the AH is closed. A lot of players would come back to game when Loot 2.0 comes online. There would be a great need for items before expansion. So this time, you should not sell items. Instead, you should buy cheap items in the auction house and then sell it in Loot 2.0.
I would not recommend Gem investment. Perfect Star Emerald has increased to 9,7000,000 D3 Gold after the expansion announcement. But now it has come back to 8,500,000 gold. With the low price, you can buy a few gems for you own use. But you should not stock them. The 15% transition fee would make you own little. In the end, the auction house wins.
In conclusion, stock cheap gold and items before loot 2.0 and sell them after Loot 2.0. Gem investment should not be a consideration.