The bike I bought before last I couldn't test ride it because I was fresh from a bike accident and my ankle was buggered. I saw the guy ride the bike and it looked great and he got it safetied for me. When I got it home and took it for a spin I noticed right away the front brakes had a bad pulsation. Further investigation showed both front rotors warped and well out of spec. if I even looked at the front lever the bike vibrated badly. Contacted the previous owner who claimed to have never noticed.
I test drove a few bikes for friends. One bike has NO front brakes at all. Owner didn't warn me. Another bike was impossible to shift gears.
Having said that the best bike I ever owned the owner wouldn't allow a test ride unless I gave him what he was asking cash in his hand. I never test rode it and it was fine.
.....well it did have a slight misfire, but fresh plugs fixed that.