Since sandbox solutions are no longer supported in SharePoint Online how can a visual Web Part that will be made available on the SharePoint Store for purchasing be created?
There's no real concept of Web part in SP Online.
You have two alternatives:
- Create a SharePoint add-in containing an app part. But technically app parts have nothing to do with Web part. They're purely declarative (in a simple XML file) and are actually a simple IFrame pointing to a configured URL external to the current site.
- Use the new SPFx (SharePoint Framework) tools to create a pure client-side "Web part". See this link.