During a pre-buy and annual condition inspection of an amateur built Searey, the mechanic advised the pilot that the airplane's landing gear was out of adjustment. According to the airplane's maintenance records, the mechanic removed two bolts from the gear retraction drag links that were "too short," and replaced them with longer ones. He also found that the bolts were hitting the over center links and "repositioned the washers for more clearance." He then performed a retraction and extension test of the landing gear. Two landings later, the left main gear collapsed. The airplane slid off the side of the runway, struck a taxiway sign and was substantially damaged. During a postaccident examination by the airplane kit manufacturer, it was discovered that the "lock arms" were carrying the landing gear loads instead of the flat portion of the "angle" assembly, which normally rested against a wear pad. Further examination revealed that, sometime prior to the accident, the bulkhead upright tubes had been bent outwards and the forked end of the hydraulic ram was out of adjustment and could not fully retract, keeping the knee joint from traveling into the over-center locking position.