The self-diagnosed client is the person who comes to you with a problem, a solution, and they only need you to implement that solution for them!
Does that seem right to you?
Consider the following scenario. A person goes to the doctor; this person knows the problem; knows the treatment and only asks the doctor for the medicine they want. In most places, it would be a case of mal praxis for the doctor to accept the self-diagnosis.
At the very least, the doctor would ask questions to confirm what the real problem is and confirm the diagnosis, right?
When it comes to online businesses, the issue is not that clear cut, and it depends on what you are selling.
If what you are offering is creative, custom made solutions, then a self-diagnosed client is a disaster waiting to happen. You cannot know if the two of you are a good fit if you have not followed your own diagnosis process that will enable to serve the client best. The client doesn’t know what they don’t know… they have a blind spot. If they knew everything, they would not come to you for a custom made solution. So it is your job to at least confirm the problem and what they think the best solution might be :).
In case of a more standardized offer, then it is indeed up to the client to make up their mind if your offer is a good fit for what they want to do. You can, of course, help make up their mind with excellent communication about your product or service.