According to the pilot, the builder/owner of the amateur built RV-7 hired the pilot, who was also a designated airworthiness representative, to inspect the airplane and to perform its first test flight. During the preflight inspection, the pilot did not remove the cowling to inspect the engine compartment. The pilot completed the test flight and returned to the airport for landing. On final approach, he adjusted the throttle, but the engine did not respond and he could not increase or decrease power. He then extended the flaps and "slipped" the airplane, but was "going too fast and was too high" to land. He turned the airplane to land on another runway, but during the turn, the airplane "lost altitude" and impacted the ramp area. An inspection of the airplane following the accident revealed that throttle control was disconnected from the fuel servo. The throttle linkage connecting bolt and several washers were found loose inside the engine cowling. The corresponding castellated retaining nut and respective cotter key were not located. During an interview, the owner/builder stated that he "may have forgotten to install the cotter key."