I didn't actually work on the systems that score reviews and filter out certain ("not recommended") reviews, so I'm not sure what signals went into them. I would guess recency, photos, account age/reputation, etc.

I had some other reasons for believing it was impossible for sales reps to manipulate review scores, but my memory is very foggy since this was ~9 years ago. Sorry if I made it sound like I had more insider knowledge; I edited that out of my comment.