Be very careful with this one! It is more complicated that you think! Read the fine print on the Air Carrier Certificate. An Air Carrier certificate has no monetary value. It is non-transferrable. If an air carrier desires to sell company assets, such as an aircraft, hangar etc, that may be permissible with certain restrictions. It is required that an air carrier has at least one "exclusive use" aircraft. That means that at least one aircraft must be owned or leased and it's primary use is for air carrier operations. The FSDO evaluates all aircraft leases to ensure that this "exclusive use" requirement is met and that "operational control" is maintained by the air carrier. Simply "buying" a 135 Certificate is not legally possible. Recommend you research 14 CFR Parts 119 and 135 very carefully and also FAA Order 8900.1. Typically if more than 50% of the required management personnel at an Air Carrier changes at one time, a re-certification may be required.