Thinking it over, I realize sometimes I use (2), which emphasizes 买(mai). Examples:
我給ni mai 一件(blouse)了. - stress on 买(mai). Implying you don't need to buy another one, or I don't want to buy another one for you.
Or if stress on blouse, the sentence is to inform you that I WON'T buy a T-shirt or a coat but WILL buy a blouse for you, in spite they are all on sale.