And why someone has to lose more than 900 $ by selling your friend the 5btc for 20k?
dummy you dont lose $900 by selling, you sell for less than the other seller because you are a miner wanting to liquidate bitcoin for cash - which you then buy more bitcoin with for less than you sold, same day!
lots of people do this on local bitcoins - how and why the hell would anyone sell bitcoin ever in the first place?
worthless comment
The dummy here is you.Why do not you sell your friend your bitcoin ? I'll tell you why