Once you want to sell your cryptocurrency for fiat, how would you do it?
Would there be any liquidity problems with semi-big sums e.g. 20.000$?
There are quite a bit of methods to do this.
The fastest i think would be to cash out on coinbase. The fees will be quite high though.
Personally if I were to cash out I would use the most trusted exchange sites. (Bitfinex, Kraken, etc.). Though I suggest withdrawing your funds as soon as you've sold your crypto as there are chances of them being hacked.