Is there any exchange where I can place some money on the books, and short sell Bitcoin?
I think Poloniex may have that feature. I used Poloniex a couple of years ago and they had margin trading but I never used that. I cant say for sure if it included short selling or if it was just buying on margin.
Another option is shorting using a Bitcoin ETF. I think there are a couple of exchange traded funds through regular stock market trading available.
never knew about it(poloniex's Bitcoin margin) ?
as far as i remembered,they're only offering altcoins Short and Long position on their margin.
(i used it a few months ago from 2015 until 2016).
from my own experienced,they're only offering us a 'Lending' system instead on Bitcoin margin(short and long).
if you want to do that i have a few exchanges that may suit you well;
1.Kraken
2.Huobi(my favorite,i used it in 2016)
3.Bitfinex