the problem with tripadvisor ratings is that anyone can leave a review, even if they haven't been somewhere or stayed there.

At least on ebay you need to have had a transaction with the person but by leaving it as voluntary, you are more likely to get people who are unhappy giving feedback, as a lot of poeple in uneventful transactions just move on to the next one.

You then get the problem of smaller sellers becoming discouraged and leaving the site which then reduces the variety and damages the 'buyer experience'