I admit that I'm unusually repelled by the idea of buying a new . . . anything. The fact that the act of driving a new bike off of the dealer's lot is economically identical to gathering a hard-earned pile of cash and throwing 10-50% of that pile into the wind may forever keep me from owning another new motorcycle. My last experience with this form of "investment" came in 1974, so I've been suffering this trauma for a long time. That's all I'm admitting and you'll have to live with any other remorse you think I should suffer.
All that said, buying used bikes is another form of self-mutilation. Over my 35 years of buying used vehicles, I've formed a collection of rules that, if I followed them, could prevent a lot of the usual used-bike/car misery. The first of those rules is "never buy a motorcycle from a kid."
If you force me to define "kid" in chronological terms, I'd have to arbitrarily say anyone under 40 is, more than likely, a kid. However, I've known 15-year-old adults and 65-year-old kids. So numbers don't do this psychological defect much justice.
Kids are destructive little monsters who think their motorcycles (and cars) are educational toys. There's nothing wrong with that logic, until the little motorcycle mangler decides to sell his &*()*#$#%@ Erector set with all the missing, stripped, bent, and broken bits "as is." It's only when a real person gets stuck with a vehicle that was previously owned by the Kid that a capital crime has been committed. Unfortunately, the capital or corporal punishment usually gets played out on the buyer, not the &*()*#$#%@ Kid.
The most fatal flaw in buying a bike from the Kid is that the little dweeb thinks he's a better engineer than the folks who designed and built the bike. If it were true, this would be a more than typically pointless Geezer rant. But it's not. The kinds of things most often "re-engineered" by the Kid are exhaust systems, handlebars, lighting, threaded holes (especially sparkplug holes), brakes, suspension parts, fuel systems, and critical bits of the power train. Not a one of these areas were easy design tasks for the skilled engineers who built the bike. Without exception, the Kid will whack away at any one of these areas without a clue in his head or a skill in his hands.
When I'm shopping for a used bike, comments made by the current owner about shade tree work done in any critical area of the motorcycle's mechanics becomes a deduction in my valuation of the bike. In my best moments, I'm ruthless about taking those deductions from the price of the bike. In my usual moments, I'm not nearly vicious enough. Most Kids won't consider messing with a motorcycle unless they have 1) already crashed it or 2) have found an easy way to really mess up the bike's operation or 3) Daddy gave them a pile of money for Xmas and they want to "decorate" the motorcycle with useless crap that suddenly became affordable. Knowing this, you can be realistic about the damage done and the resulting price deduction. However, it's hard to keep all that in mind when you just want to buy a bike for cheap and go for a ride.
Even if the seller is 95 years old and only rides the Goldwing at Shriner parades, you should probably make the paranoid, or conservative, assumption that he's the Kid. With that thought in mind, start from the front of the bike and work your way back, looking for mangled bolt heads, loose stuff, non-stock stuff, and beer cans hammered into clutch plate shims. When you finally do get to ride the bike, be critical. Don't assume that any odd quirk in the bike's performance is something you need to get used to, assume it's an introduced design flaw, courtesy of the Kid. Don't even consider hoping the quirk will go away after you ride it a while. Fix it before it fixes you.
From here, I'm tempted to go into a marginally rational rant about aftermarket pipe, carb tuning kits, and suspension modifications that turn the bike into a self-destructing, back-breaking vehicle that will cripple you in any number of ways. But I won't because I've already been there and it makes me crazy. I will suggest that every fastener between the top of the tank to the manifold clamps is probably cross-threaded. Buy stock in Emhart Fastening Technologies (the folks who make Heli-Coils) before you get started.
Even better, do what I hope I will do the next time I come upon a Kid's bike being offered for a great price; walk away and don't look back. It's not worth the hassle or hazard. Repeat after me, "I've been here before and I will never do anything that dumb again." Or do like me, forget that mantra and spend most of the riding season returning your bike to the condition real engineers intended.