I´ve noticed on coinmarketcap.com that sometimes the BTC is lower than the Dollar (USD). It doesn´t always follow that price.
CoinMarketCap uses an average data from multiple exchanges. You better don't use CoinMarketCap since it a scam(Google "CoinMarketCap scam" you should able to find trustable sources)
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.
That is mean the coin you bought decrease in a pair of Bitcoin, but the BTC/USD price is creasing significantly, so it makes your capital is profiting in term of FIAT.
So what do you guys look at when trading ?? The USD or the BTC
It depends, if I want to increase my Bitcoin capital, then I trade with BTC pair. In the other hands, I use USD pair.