Professionals have a reputation, which should reduce bias or influence. If Zagat was known to take bribes, they wouldn't be around for very long.

It may happen, but is less likely to happen than non-professionals. You have a much larger issue with selection bias for reviews, and a much easier way for vendors to bias them. I've been in shops that insist, rather persistently, that I write a good review for their store on their computer, at the shop.

Great points! The issue of selection bias is very real on many review sites. Especially with the volume of reviews being so low for most products and services.