Depends....
Maybe he is just real protective of his old, nice condition, scooter!
This is ALWAYS a touchy subject where I live.....even new scooters cannot be test ridden here unless the dealer is really motivated to sell. Most often....no test rides.
Maybe meet him in a large parking lot....have him put it through its paces. Hard accelerating, hard braking, sharp turns. Sit it, grab the.bars....look for play in the steering....front and rear wheel play ----side to side. Look for abnormal tire wear patterns, scored brake disc, lights all around. Make youself a pest...but check it out as well as you can for half an hour. Repair or service records? etcetera, etcetera.
Might not be a red flag.....just a careful owner like many of us.
Stig