低气温和气温低 are not the same. and you know, black cat and the cat is black are different. 低气温 is a phrase（low temperature）, but 气温低(the temperature is low) is a 完整的 sentence.
when sb ask you "what's the weather like today?", you may say "气温低/今天气温低"， it is seldom that "低气温" is used to be the answer, it's not good, although people can understand you.
买东西了，买了东西，买了东西了, the are almost the same. 极细微的差别对于初学者来说很难，难以掌握。比如，你进商店，可以对店员说：“买东西了！”，此时，相当于hello.