Even if a knife sharpener has a stellar reputation, anyone can have a bad day at work. Conducting research on a person you are considering hiring to do a job certainly won't guarantee a positive outcome, no matter how great their work history is.
I think the sign of a true professional is not so much their ability to do a perfect job every single time, but rather their dedication to customer satisfaction and their willingness to make it right when the customer is unhappy. If they screw up the job, and the customer is unhappy, then they should do what it takes, within reason, to make the customer happy.
Of course the customer has certain responsibilities as well. Like making it very clear up front what they want, and what they expect.
I have always done my own sharpening, but if I were going to pay someone to sharpen my knives I would make it very clear to them exactly what I wanted- how steep a bevel on the edge, remove as little steel as possible, no scratches on any part of the knife (if it's appearance mattered to me), be careful not to round off the tip, etc, etc. I would also want to know exactly what they were going to use to sharpen my knives (stones, belt sander, grinder, electric sharpener). And if I didn't like their attitude regarding how I, as a paying customer, wanted the job done, or if I didn't like their method of sharpening (belt sander, grinder, electric sharpener) I would not give them my business.
As a customer you really have to be specific about what you want and expect. Otherwise the guy doing the job might think you really aren't all that particular, and instead of doing a highly refined job, he might just get it done.
A lot of people, including some professional knife sharpeners, think that sharpening a knife means nothing more than putting a sharp edge on it, with no concern for appearance or how much steel they remove. And if the knife they give back to you is sharp, they might feel they have done their job, especially if all you told them was "I want you to sharpen my knife".
Like I said, you have to make it clear up front exactly what you want. And the clearer you are, the more legitimate your complaints will be when their work doesn't meet the requirements you gave them.