if you think your review was fair - the product was okay, not too expensive but nothing special, or something along those lines - then it's fair to leave it as a warning to other buyers. Not a warning to avoid the seller, just a warning as to what they can expect.
But be sure in your own mind that your rating was right. What could they have done to merit four or five stars? If you think they had room for improvement, then three is fine.
And if there's room for improvement, they should concentrate on improving rather than bribing.