Buying a house, under annoymous?

So I know this guy and I've known him since I was a little girl, well he owns a duplex in which I have lived in 4 times. I know he would sell his house for a good offer but the question is, how do I go about negotians without him knowing who it really is (me) that wants to buy his house. The reason I ask is because we had a bad run in with each other and he dislikes me a bit now.