How can I assure all contracts are properly endorsed in a buy-sell of an independent pharmacy operating as an S-Corp? goal

Your question doesn't make any sense. Contracts aren't "endorsed." If you wish to have a valid buy-sell agreement with respect to your business, you need to consult with an attorney who can properly advise you and then draft agreements which, when properly executed, will carry out your objectives. Part of the role of an attorney is to supervise the execution of the agreements to make sure they have been adopted by the parties with the requisite formality in order to be enforceable.