I have never left anyone negative feedback and I have over 40 POsitive feedbacks. My questions is: When should it be left.

A few weeks back I posted about someone who needed a watch for Thankgiving and they told me they were going to paypal in a few minutes, so I left my office and went home to get the watch so I can fed ex out and they could have for Thanksgiving, then they never paid and never got back to me. I should have left negative feedback, but then what would stop then from leaving me neg feedback and ruining my score?