Actually, I would expect the ratings to be reasonably high.
Most people will already make a judgement on the card based on the preview, the photos and even the written reviews before they buy. So they invariably expect to like the card. And hopefully that is what happens.
So I would expect the average rating to be high, because no one will buy a card they don't think they are going to like.
In saying that I have been dissapointed with cards and given low ratings, but that is just my style. It is not better or worse than anyone else.
The exception is the members of course, who just receive cards, they may receive cards they don't like and rate accordingly.