1) Yeah I know there's few retail brokers that atleast give you books (ladder) and ability to match with other customer quotes, that's great if you have that but it's still very different from say futures or (based on what I've read) reuters/ebs. And by different I mean that if you are doing trading in volatile low liquidity situations, there's less risk of what happened to op if you say traded through futures/fop, swap execution facilities etc - largely because with MM-broker you get quotes that are not real but something the MM-broker assumes they can hedge if need be without losing. If it turns out they would lose they'll cancel your profits and invent simplistic explanations. Above I reasoned more detailed explanation for why they have a case for cancelling just the profits. Some other posters had similar arguments too I noticed (eg. just the profitable trades were out of market)
2) I admit that I haven't written the algorithms behind the bucket shops books & hedging so it's possible I am wrong and they somehow are able to perfectly hedge all the time...
3) The part about understanding is that if your profitable strategy is such that it eats your counterparty for lunch and you have always the same counterparty then that counterparty either defaults or stops trading with you. What I am trying to say that if you are getting profitable and suspect it could be due to some sort of arbitrage, understand what you are doing and "stay under the radar" instead of trying to kill your own golden goose so to speak.