1,From where to "BUY" :
In most of countries bitcoin is not legal I mean in India,Pakistan etc etc because of fear that goes in the head of bank authoroties.SO,first choice should be localbitcoin.com (i will then write about some mistake that we make when we buy btc from anywhere ,that hidden margin people did not notice)
2,BTC/Any altcoin:
Just go to coinmarketcap, open the coin which you wanna buy ,scroll down the page that you open and you will see this....
NOTICE: Pick the best exchange that is running becasue when you buy with satoshi number of btc that PROGRAMMING of any exchnage play a vital role in quick buy and sell
3,Where should we SELL:
we have selling option also in localbitcoin.com so first priority should be this but there are other options also like coinmama,satoshitango etc
EXTRA POINT=>what to do when market is BULLISH:
Just shift all your coins to Tether(USDT), if you can leave market then just sell .