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I know 下来 so far as complement for verbs of motion, e.g. 跳下来，走下来，攀下来，掉下来. In them, the first character of the complement conveys the meaning "downwards", the second the point of view of the speaker "towards him".
Obviously,下来 in 买下来 has a different meaning than obove, how is it used and how is 买下来 different from simply 买 ?
Do these sentence differ in meaning?:
把这些书买下来.待会送到我家去
把这些书买.待会送到我家去
下来 puts an emphasis on the fact that the action is completed.
雨停下来了。The rain (has) stopped. （It's over, it's not raining anymore).我们把休假的地方决定下来了。We have decided where to go on holiday. (The decision is completed, it's done/settled) 他慌乱平静下来了。His frenzy calmed DOWN. (May be this is the closest example in English where we use the same directional complement. Once he calmed down, his frenzy was over, so this 'happening' got completed. It's difficult to translate your example sentences in a way that points out the difference, since buying something is an action that is always completed (you either buy something or not).
indicating success， cf。实用现代汉语语法 ５５５页 （下面所有文字都关于＂下来＂） ５。结果意义（denoting result）（三）：表示完成一件费时、费力、需要克服一定困难的动作行为 （expressing completion of some time consuming，strenuous activity involving certain difficulties）
（１）一天的重体力劳动干下来，身体好像散了一样，一步也不想走了。 （２）三年中文学下来，他的进步是明显的。
因此有某些使用者建议把＂把这些书买下来＂翻译成＂manage to buy these books＂。
it seems this meaning may also apply to 2 of earlier answer ＃１,
on the other hand 1,3 of answer ＃１ seem to correspond to (请参考比如说＂外国人实用汉语语法）： 下来 可以表示动作或状态由动到静，由明到暗，加深程度等逐步变化的过程 expressing the gradual change from an active state to a static state or from brightness into darkness. 例如：天黑下来了。 火车满满停下来了。
I think your case is among those occasions when 下来 is used to imply that to make the action is to grasp an opportunity.
