A better business would be to sell an API that is legally sound for signing documents. Then everyone could implement the functionality in their own tools as a feature.
Docusign effectively does this with all of its integrations. If you run on Salesforce, it's really well integrated. And if not, they have a great API. You can programmatically generate an https link and send that to your customers as part of your existing workflow. You email them a link or even just open up a new tab / overlay a browser. They can log in with various SSOs, they sign the form, then come back to the next stage in the workflow.
What most people posting here don’t realize is that they couldn’t replicate docusign’s 50 billion dollar business with a copy of the source code and a time machine.
Edit: see https://www.docusign.com/products/apis