You absolutely can! As long as the clause doesn't violate another law, you can 100% put in a clause that says "You will continue you provide the service for 365

You absolutely can! As long as the clause doesn't violate another law, you can 100% put in a clause that says "You will continue you provide the service for 365 days from the date of purchase." Typically the rest of that clause would specify the penalty for the purchasing company, which could range from monetary penalties up to cancellation of the sale, which would immediately revert all assets the company owns to the original owners.