Your answers might go something like this:
I am sorry officer, but I legally sold my guns to another citizen.
No, I did not keep a sales record.
No, you may not enter my home without a warrant.
Thank you and have a good day [close door].
Great response! The advice I was given was to be pleasant and as honest as possible when you talk to the ATF. They got my info from 2 local gun shops where they entered my information into their laptops. If you don't play nice they tend to threaten and harass. I know of at least one other person who got visited by them as well.