Usage patterns
〈事〉の商いをするto run a business dealing in ~
My grandfather ran a fish shop business.
商いが繁盛するfor business to thrive
The new shop's business is thriving.
商いを始めるto start a business
He started a small business.
商いが落ちるfor sales/turnover to drop
Sales have dropped due to the recession.
Expressions
商いは牛のよだれone should not rush to make a profit; business is best run when sales flow like a cow's drool