You need a way to narrow down, among all the people using Mechanical Turk, people who actually fit the user's target demographic. You need this right from the get go.

e.g. What people in India think of my craigslist add for something in North Carolina is totally irrelevant.

Also, I agree with another poster, who basically stated that you should make a generic comparison service, and let people compare anything, whether they be two domain names, two versions of a site, or whatever.