Managing users for courses I sell

You can certainly set an expiration date on the Captivate output, but then that same expiration date would apply to all users that access the content.  I expect that is NOT what you are looking for because you would then need to be republishing a special version of the course content for each and every client or user.

There is nothing in Captivate that allows you to control user licensing of the course.  Most of the time this handled by the Content Management System (CMS) or Learning Management System (LMS) via an e-commerce plugin of some kind.  If you have your own LMS, and the only people that can view your course are getting it from that LMS then you can know how many users you have and who they are.  However, if you allow your content to be loaded onto other people's LMS then things are much more difficult.

One solution you might investigate is using SCORM Cloud's Dispatch service.  The content lives on their servers but can then be dispatched for view in other LMSs.  I don't know how flexible this works for what you want to do, but I strongly suggest you investigate it.

The only other solution I know of would be to have custom programming done that enables you to get data about users interacting with your content separately from the LMS where they are viewing it.  I know my son Tristan the WidgetKing has done this for a couple of his own Captivate clients.  But you'd need to talk to him about the feasibility and costs.  You can contact him directly from the Infosemantics.com.au website.