You don't rob an online casino with martingale system. You will just lose your money (unless of course you have unlimited balance on your account).
And if the casino doesn't have a table limit. But still, you could make a few hundred bucks in the short run if you are lucky :D
Perhaps, but house advantage ensures you will keep losing in the long run. The only thing keeping you from a guaranteed win in the long run is the table limit. I ran some simulations a couple of years ago to test some strategies - it was quite shocking to see how common losing 17+ in a row is. Losing 6 in a row is bound to happen every few rounds due to martingale system relying on a lot of 'small' rounds to scrape in winnings.
A link to wiki article about the Martingale system involved: