When I pulled up to my buddies' shop in San Clemente, CA the other day, there was a young Marine standing outside who had just rode in on the Star Bolt he has just bought from a friend. The good news was that he picked up this awesome two-year old motorcycle for $2k. The bad news was that this thing had been absolutely butchered by whoever he bought it from, and he rode it in with the rear shocks ZIP-TIED on. Not only that, but they had removed the spring from the shock and drained out all of the fluid to basically turn it into a short strut. OHMYGOD. How this kid managed to make it the fifteen or so miles from base to the shop here is beyond me, but I am sure glad he did. If for no other reason than to smack him upside the head and let him know how incredibly unsafe that monstrosity he is riding is.
The terrible truth of the matter is, someone thought they were improving upon this bike when they did this. Whoever it was then sold it to his friend with a totally clear conscience, thinking he would be safe riding the thing away.
If you're buying a custom bike second hand, pick the owner's brain. Make sure you ask a lot of questions, and if you aren't getting clear, confident answers, chances are the person doesn't fully understand their bike. If the owner was the one to do the customization, and is not a professional fabricator or motorcycle mechanic, you'll want to be even more skeptical of their work. Not to say that there aren't awesome and dependable customs that were done in some guy's personal garage, but they're a harder to find than something like this death trap.