With 12 btc, what I would do is :
Sell BTC around 900$, buy back more BTC around 800$, sell a part at 850$ and another part at 900$ and start again the loop.
It is a good moment now to do this kind of trades due to the votality.
Unfortunately the market of bitcoins don't just go around that price range. We're not stuck in a loop of 800-900$ range but we might spend quite some time here. Eventually, the price would be more than 900$. What if you just sold your bitcoins by that time? Are you going to buy bitcoins knowing that you will lose money if you do so? I don't think so.
Price volatility might be taken advantage of. But not all of the time that particular strategy would work. Trading is much more than buying at x price, selling at y price, and then waiting for the price to go back to x then sell to y price.