There is currently no off-the-shelf feature to integrate a seller's external calendars (E.g. Google Calendar, MS Outlook) to your marketplace. You can upvote this feature or request it in the community section of Arcadier Support.
However, if the APIs for the external calendar are available, it is possible to build an integration with our APIs. This can be done with the help of an external developer. You can check out our full-stack Expert Partners here.
Do note that you would have to be on our Growth, Scale, or Enterprise package to get access to our APIs.