It's not just that. Sometimes, they genuinely don't carry what I'm looking for. If that's the case, then the response is simple:
"I'm sorry. I'll make a note of that, and pass it along to the manager." And then you take out a pencil and notepad and jot down the name of the thing I was looking for.
This makes me happy because it shows that you're listening and taking my needs and requests seriously. Sometimes (and this has actually happened), it makes me even happier because management will actually decide to start stocking the thing, so that I can find it there on future trips.
Paul
