I'm guessing what you are looking to purchase is something related to link building. If it is a cheap service, I would be very, very careful.

There are a couple of issues with reviews on this site. First, there are tons of just outright fake reviews and reviews from members of circle-jerk groups. WF tries to crackdown on that as much as possible, but honestly it is just difficult. It would be nice if they would institute a "verified purchaser" kind of thing like Amazon does, but even that can be gamed.

The other thing I would be careful about with the reviews is reading carefully exactly what they are reviewing. Many times with SEO gigs and link building jobs, the reviewer basically just says something along the lines that the work was done as promised and in a timely manner. What you do not see as often are stories about rankings improving or repeat orders.

Sometimes a customer does see a ranking improvement, but are they still ranking 6 months later? A year later? They don't often come back months later to mention how their rankings tanked.

Lastly, with some of these cheap services, the customers are buying 4 or 5 different services at the same time. They have no idea which one really helped or hurt their rankings. The waters are too muddy.

Make sure you understand what kind of work they are promising to do. If you don't know enough about SEO to know whether or not that type of work will help or hurt your site in the long-term, ask someone who does know. It's better to be too cautious than to let some knucklehead damage your website.