If what you're saying is true, then every time an ETH is bought or sold for BTC, the volume is attributed on both sides of the ledger (i.e. it's ETH volume and it's also BTC volume). That's not how it works. CoinMarketCap treats Bitcoin as the primary crypto currency
I'm not sure how CoinMarketCap is designed to works. What I described is the correct way of how volume should be accounted for in general. Volume doesn't care if it is a sell or buy transaction. All BTC transactions are accounted in BTC and all ETH tranactions are accounted for in ETH. It doesn't matter if they are sell or buy orders.