Originally 商 meant a traveling merchant and 贾 a settled shopkeeper. Together 商贾 is a literary/classical term for merchants in general. Note: 贾 is read gǔ here, not jiǎ (as in the surname).
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商贾 is a literary/formal word rarely used in everyday speech. Modern equivalents are 商人 or 生意人. You'll encounter 商贾 in historical texts and formal writing.
