We know that there was no binance at that time but there were other exchanges like Mt Gox and later poloniex was one of the biggest crypto exchange.
People use to buy/sell bitcoin and altcoins on these exchanges. Today there are so many exchanges for the spot (https://www.coingecko.com/en/exchanges) , dex (https://www.coingecko.com/en/dex) and derivatives (https://www.coingecko.com/en/exchanges/derivatives).