This is a multi-pronged question. It may not seem like it, but what you're asking for is really quite complicated.
You need a system to:
- Handle purchases and credit card transactions
- Handle renewals of intervals on your chosen timeline
- A web-interface to your DNS nameservers so the user can manipulate their own records
and, most importantly, you need a really, really good lawyer to do you an iron-clad terms of service when you're purchasing, because nomatter what you say, YOU still own the primary domain, which means that YOU still own the subdomains.
For example, what happens if you forget to renew your domain? All your subdomain users will be left out in the cold and they'll want their money back, and possibly even compensation.
You will also want to read the terms of service for the purchase of your domain to make sure that this is even legal under the terms of the TLD (
None of these issues really fall under a typical "server administrator" role, it's more of a business logic/programmer role.
The only part of your question that is appropriate for here is "How can my customers manage the DNS via a web interface", in which case there is an appropriate question that has already been asked for that here.