You're right, and sometimes, the bike it's already set up, all you need is in there already installed. The always first thing to ask is, the issue, why the owner wants to sell it in that condition. But honestly in our country, its really hard to buy a cheaper bike and in good condition, first come to your mind is, this bike might be stolen one.
As for buying a stolen bike, I always demand for the receipt when the bike was purchased. Also, I try as much as I can to make things work pretty well in the sense that I buy from those that I can trust.