买 vs 购 vs 购买（近义词）

1)All the three words mean "to obtain something by paying money".The difference is:买 is the most colloquial;购 and 购买are literary.Besides,买 and 购 are monosyllabic and therefore more free in collocation with other words.But since 购 is literary,its collocation is somewhat limited.购买 is disyllabic and mostly collocates with disyllabic or multisyllabic words.Moreover, each word must be used symmetrically with its antonym.e.g.:
2)The antonym of 买 is 卖；the antonyms of 购 and 购买 are 销 and 销售.
3) In the above examples,①and②are colloquial,so 买 is used and not 购 or 购买；③ and④are literary and contain set
phrases,so 购 is used not 买 or 购买.In example⑤买 can be used instead because it is very applicable,but 购 cannot be used(compare:购一物，购买一公债).In example⑥，购买 and 销售 are symmetrical,so 买 and 购 cannot be used instead.
4)购 can also form set phrases,e.g.急购(urgent buying),求购(need buying),函购(buying by mail),邮购(buying by post),etc.which can be found in advertising language.