buy: 普通日常用词，既可指日常随意购物也可指大的购买。vt./vi.购买 收买 买；n. 购买 便宜货
purchase: 正式用词，指大宗购货或购买重要东西。无感情色彩，强调物品的购得。
buy 的相关短语：
buy up 尽量收购, 全部买下
buy off 出钱摆脱, 向 ... ...
buy out 买下 ... 的全部股份
buy in 买进, 购进一大批
buy back 买回, 回购
buy over vt. 收买(贿赂)
buy on tally 赊购
buy cheap 廉价购买
good buy 便宜货
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