Over the years, responsibilities in the face-to-face therapeutic alliance have become fairly well defined by a body of law, regulatory agencies, and professional associations. The American Psychological Association (APA), for example, maintains a code of ethics designed, among other things, to 
protect client welfare and promote the treatment process. Most states require that therapists be licensed and that they adhere to a code of ethics such as that of the APA. Many states, including Maryland where I work, go further and require that therapists continually develop their skills and knowledge base by participating in professional education programs.
The Internet presents a rather different set of circumstances: it remains a dynamic frontier that continues to evolve. As things are now, it can be a challenge just to know who you are communicating with; it might be a trained therapist, or some alien with a web site. 
Even if you know you are interacting with a trained therapist, the actual services they offer via the Internet will necessarily be different from those offered in a face-to-face setting. Another source of uncertainty: the significant differences between face-to-face psychotherapy and online therapy almost require slightly different legal/ethical codes for the two environments. The specific manner in which locally established legal and ethical guidelines for psychotherapy are applicable to non-local, therapeutic Internet interactions is far from being determined.
What does all this mean for you, the potential online therapy customer? First, it suggests you should verify that a provider is who and what they say they are. You might, for example, check out a provider at Metanoia or a similar credential-checking site on the web. Better yet, check out the provider yourself. If the provider claims to be licensed professional, you can contact the licensing agency and verify their license. In addition, you should carefully read how a provider describes the services they are offering. If you are not clear about something that is relevant to your situation, ask before you sign up. Save the response you get. Then, expect a provider to deliver the service they promise.