buy, tradesman
賈 centers on commercial exchange: buying and selling, or the person who does it—a tradesman. The two given meanings are two sides of the same mercantile activity.
賈 combines 覀 (a net or cover) with 貝 (shell, money), suggesting covering or handling money in trade. The exact historical development is uncertain, but the components point to commercial activity.
The top 覀 looks like a net or cover, and the bottom 貝 is a shell (ancient money). Picture a tradesman covering his goods with a net at the market, ready to buy and sell.
For コ, imagine a tradesman buying a whole 個 (こ, individual item) of goods: 個 -> コ, and the shell money 貝 changes hands.