What does that mean in concrete terms?
With the purchase of a software licence, the buyer acquires the right to legally use the purchased copy - and to resell it if necessary. As a rule, this right also includes existing update and downgrade authorisations. It is irrelevant whether the software was provided on a data carrier or via download.
Clauses in licence agreements that are intended to exclude resale are invalid. Copyright naturally remains with the manufacturer, but this is not affected by the used software trade: It is not a question of unauthorised duplication or "pirated copies", but of passing on products that have already been licensed but are no longer used.