Hey guys

Just a little newbie question

I´ve noticed on coinmarketcap.com that sometimes the BTC is lower than the Dollar (USD). It doesn´t always follow that price.

I´ve bought some coins for X amount for BTC´s ... but when I want to sell that coin, the BTC value is actually lower than when I bought the coin for BUT the USD has price has grown.

So what do you guys look at when trading ?? The USD or the BTC

Thank you