Airframe & Powerplant technician with Inspection authority reported they released an aircraft for service without visually verifying that the maintenance task has been completed.

Narrative

The above aircraft was involved in a hard landing event with damage to nose LDG & supporting structure. The aircraft was recovered; disassembled & transported to repair facility in ZZZ. The aircraft was repaired & re-assembled after many months. As the airplane was approaching completion; I was tasked with conducting an annual inspection. The inspection was performed per CFR 43 Appendix D using company check list that conforms to requirements. Due to the complexity of the work that was done to restore the airplane; I had conducted the inspection as work was still going on by multiple technicians over many days. The aircraft was restored & I was informed that it was ready for return to service. I reviewed all the records associated with the repair & conversed with technicians involved. After satisfied that all work was complete; I returned the airplane to service & made log entries for the work done as well as the annual inspection. The aircraft was test flown with no abnormalities noted. Nine months later I was notified that during a pre-purchase inspection several issues were noted regarding the repairs that were accomplished. The most serious being structural fasteners that were not installed on the top floor panel to fuselage support channels & poor workmanship on several other fasteners related to these repairs. This resulted in the airplane being grounded until these repairs can be completed.The aircraft should not have been released until the repairs were completed properly. Human factors such as time pressure; lack of experience on technicians performing the work; poor communication; inefficient management / coordination of the workflow & lack of job turnover documentation & accountability were factors. My reliance & trust of persons that performed the work and communicated that with me and then where I failed to verify work completion resulted in the airplane being returned to service in an unairworthy state.Going forward; I will adjust & re-focus on my own personal minimum's checklist particularly when complex & lengthy repair scenarios are encountered. I will visually verify that maintenance task has in fact been completed before return to service.