What is the function of the complement 下来 in 买下来?
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).