I've done a very unscientific survey over the last 3 free listing weekends. On the first two I sold 8 and 12 items respectively. Once I had posted the items I immediately left positive feedback for the buyers. In each case I got 2 - 3 positive feedback, I never heard from the rest so presume all was in order. On the 3rd weekend I held off leaving the feedback until I heard back from the buyers and lo and behold I've already received 12 out of 14. I'll be doing it this way in future methinks!