As far as I'm aware it's because he hasn't "been reviewed enough for a meaningful average". It says it only shows up if a meaningful average can be calculated (it's worded a little weird) but if he needs roughly 10 reviews per month (possibly more if the reviewers don't carry much weight) then only making 28 transactions over a year+ of vending wouldn't cut it. Also, if you ever see those vendors who have excellent reviews and feedback but then one review for a relatively inexpensive product drops their feedback by quite a large percentage - they're the feedback padders whose own fake buyer account weights counted for nothing at all! Good system to have!