Thanks for your feedback. My problem is not where to buy BTC but in which platform to sell it without leaving the platform and transforming the BTC to euro or USD. This is useful when you know there will be a crash like happened last week.  For example: in poloniex you cannot do this  (USDT is not really USD and the conversion rate is quite low on polo).
Hope it is clear.
Which one use Fiat Bitfinex or Bittrex?
thanks