I teacher mine told me that 买 could be use to buy/purchase but also to "to decide to buy", or "go shopping", other teacher told me the first one was wrong.
Who is right?
买 means "to buy". But the phrase 买东西 means "to go shopping". You can use similar phrases like 买衣服, 买蔬菜 and so on to mean "shop for clothes / vegetables". You could say:
I went shopping, but the store was closed, so I didn't buy anything.
There could be a similar context where it means "decide to buy", but I haven't heard it used in that sense.
Your second teacher is right in that it normally does only mean "to buy".