I see a lot of people saying that there aren't a lot of attacks that having a 51% enables. I'd like to understand that a little bit better. Are there markets wh

I see a lot of people saying that there aren't a lot of attacks that having a 51% enables. I'd like to understand that a little bit better. Are there markets where you can short-sell cryptocurrencies? If so, mightn't you be able to turn a good profit by shorting one of these smaller coins (say Quarkchain at $30/hour with a market cap of $116M), then launching a 51% attack for a week or two, in the hopes of tanking the coins value? That would only cost you $5k per week.