And my last question is: What about withdraw fiat in the expensive exchange?
buy cheaper, sell higher and WITHDRAW USD with wire or sepa
And Margin trading? it could be cheaper, buy btc and sell margin in bitfinex or other
By far not all exchanges provide favorable conditions for fiat withdrawal especially SEPA. I'm only aware of a few exchanges like CEX.io that offers SEPA. I wouldn't recommend getting involved with margin trading without first acquiring a source for quality reliable signals. Taking on a margin is highly risky business, its known for leaving newbies broke.