The middle top is Shell, which was used as currency or money in ancient China. The bottom and sides is a net. If a person takes a net of money (shell), he must go to buy something.
The bronze script for Buy is
The net on the top, and the shell in the bottom in this character.
The big seal script for Buy is
The small seal script is
very similar to big seal script.
The clerical script for Buy is
The current traditional character (song typeface) for Buy is
almost same as its clerical script.
The current simplified character (song typeface) for the character is
which is completely changed, no any similarity to the old scripts, and meaningless. You have to memorize it.
Its pinyin is Mai3.