买卖 means "buying and selling," "trade," or "business" and refers to commercial transactions or business dealings. It combines 买 (buy) and 卖 (sell) into a single noun. It can describe both the act of trading and a business enterprise, as in 做买卖 (do business).
他在镇上做了一辈子小买卖。
tā zài zhèn shàng zuòle yī bèizi xiǎo mǎimai.
He ran a small business in town his whole life.
这笔买卖双方都很满意。
zhè bǐ mǎimai shuāngfāng dōu hěn mǎnyì.
Both parties are very satisfied with this deal.
最近生意不好,买卖越来越难做了。
zuìjìn shēngyi bù hǎo, mǎimai yuè lái yuè nán zuòle.
Business has been bad lately, and it's getting harder and harder to make deals.