I would have several questions before negotiating seriously, and I could actually be in the market for such a beast.
At what multiple would you be comfortable considering selling? If revenue has dropped 85% from its peak, have you identified the cause of the drop? Has it been steady that past five years? Do you have a record of the time spent on it, or does it just feel minimal? How much of revenue is from ads vs subscriptions? Is it sticky mainly because a user can’t export things and import them to Google Calendar or something?
Is it custom software, heavily customized software, or are you basically selling the calendaring component of something like Citadel or Horde? What languages are in use? Does the buyer get just the site or full ownership of the codebase and the rights to derive new products and services from it? Does it come with the domain and trademarks?
Are you selling outright, or are you reserving some royalty for yourself?
What does the handover look like? Does the buyer just get an email with URLs and login credentials, or do you plan on familiarizing a buyer with the whole thing?