The burn amount shouldn't be proportional to the rating.
I realize now the system described above is faulty, as the vendor must not be able to hide transactions with a low rating. Also, of course the review value shouldn't be specified by the buyer before he has received the goods. But I'm pretty confident this can be worked around with the right protocol.
There have been some behavioral economics experiments showing that people are quite willing to make sacrifices to punish offenders.