First of all you can not buy the certificate. You have to buy the company that holds the certificate. The certificate is issued by the FAA and is not the property of the company. When you do that you do have to keep most if not all the current management in place. Finding a 121 certificate will be difficult, as when airlines merge the non surviving certificate is surrendered. After all the surviving airline doesn't want to make it any easier for their competition. Depending on what you want to do, it might be easier to buy a 135 company and upgrade to a 121 certificate. But be prepared for a lot of work and effort.