Am I understanding correctly that the writer would be producing this sample with no guarantee of being paid for it? The problem with this business model is that the better writers will not do this. There is too much paid work out there for a good writer to be messing around with producing free samples. Therefore, only marginal writers will sign up to work for such a service. Good writers want a very straightforward system, and they want guaranteed payment.
Trust is an issue in this business, but in my experience, if you are a trustworthy native English speaker, it is not THAT big an issue. (I am speaking from the perspective of an individual writer. I know very little about the companies.) American clients can tell from an email or two whether they want to work with you. They look at the grammar and phrasing. And you can send them a sample of an earlier paper you've done. If that's not enough, you can talk to them on the phone or Skype and they can hear your accent. ESL students can't read as much from the informal communications, so they can be quite guarded. Generally, as an individual writer, you must depend on word of mouth to get many of these customers. When they hear about a good writer, they will pass the name along to their friends.
So in my opinion, if you are offering a good service, it is not necessary to go through a bunch of contortions, such as free samples, to earn customers' trust. If you're trustworthy, the trust kind of earns itself.
