Andy,
I have left many negative feedbacks.
I have not hesitated to leave negative feedback on about the twenty sales over the past few years that were not completed.
I went back and checked my records. In the past 3 months I have had one a month not complete purchases. Two involved Metrocards and one stiffed me in February for chips (justinsprncess69).
It turned out that all three became "Not A Registered User" before I could leave negative feedback.
If someone did leave me negative feedback, I do not think it will be the end of the world. But if someone has a high rating and a lot of positive feedback, I do think it at least shows that they try.
Thanks--Alan.