Your install base is the business. The new sale is just how it started
A reseller's first sale to a client is the least profitable thing that will ever happen with that account. The licence margin is set by the publisher. The money is in what follows: the implementation, the data migration off the old system, the report customisation, the training days, the second and third user licences, the add-on the client needed once e-invoicing thresholds moved, and the renewal that arrives every year whether or not anyone works for it.
Which is why the most expensive spreadsheet in most partner firms is the renewal tracker. It is usually accurate, usually maintained by one person, and usually consulted in the last week of the month. A client whose subscription lapsed in February gets discovered in July when they ring about something else, and by then they have either paid a competitor or decided they can manage without you.