Further Explanation - How a criminal steals your bitcoins after they have hacked your account.

You choose to have bad security (solutions mentioned above)
Your account gets hacked.
Hacker waits for you to start trade
You pay
Seller releases bitcoin to your wallet (sellers have no control over what address it goes to. They don't even see your wallet address).
Hacker sees you have bitcoin
Before you can do anything, hacker transfers bitcoin from your wallet to theirs (that is the second transfer you are seeing)