Not really, if someone had the insane power of 60% mining power, the probability of mining 6 consecutive blocks would still be 0.60^6 only ~4.67%, but this falls exponentially, so if they wait a few more minutes and have to reverse one more block, that's 2.8%
It's just very unlikely this could work. Possible, but very unlikely.
Edit: Also, if you have a very large amount of coins, you probably want to sell it off slowly, instead of tanking the market in one go. This makes it even more difficult since if you're caught reversing the first few transactions, your account will be flagged.
It's just very unlikely this could work. Possible, but very unlikely.
Edit: Also, if you have a very large amount of coins, you probably want to sell it off slowly, instead of tanking the market in one go. This makes it even more difficult since if you're caught reversing the first few transactions, your account will be flagged.