Quick turn around time.
You would want to work with groups or individuals who can write your articles as fast as possible.
If you sign up with individual service providers, they should be able to deliver 20-35 articles per week.
If you sign up with group of SEO article writers, they should be able to offer you with 100 (minimum) per week.
Proven track record.
Sign up with those people who have already made a big mark in this field.
Ask for their online portfolio and the list of their clients.
Call some of them and inquire about their experiences.
Go with those ones that were highly recommended and those who really care to give you 100% satisfaction.
Honesty and integrity.
With so many people who are out there to scam you, it's important that you work with those that you can trust.
Take a look at their website before you sign up.
Does it communicate integrity and honesty? Then, do your research.
Victims of scammers or unreliable service providers usually share their experiences on relevant blogs or on their websites.
Your service providers must be committed in giving you your money's worth.
They must be willing to go to great lengths to make sure that they'll be able to satisfy you.
They must not only offer you with great, highly optimized articles but great customer service as well.
They must always be available for your questions and complaints.
Lastly, they must be willing to listen to you and they must value your thoughts and ideas.