Like so many things in life, buying a used motorcycle can be approached in one of two, very different ways. You can treat it as yet another character-building exercise, much like cleaning a spittoon or eating somtam (spicy salad) in the morning. Or you can make it a proper adventure, something worthy of your best effort that will get you a suitable reward in return, a splendid, new motorcycle, the one you really lust for.
Fortunately, buying a used motorcycle has nothing in common with buying a new car, one of life's most enervating, soul-destroying events. You might have to deal with a private owner who's certain to his very marrow his steed is the finest in the whole world and deserves a suitably lofty price, no matter what heinous things he's supposedly done to improve it. Just beat a hasty retreat and try not to get any of his copious spittle on your raiment's.
There are three crucial parts to the used motorcycle buying process. First is finding what you want; second is seeing if it's worth your hard-earned wages; and third is sealing the deal.
We'll assume for the sake of our story that you've figured out you want a streetbike, and which one. As far as finding your intended, you have more choices than ever. To start with the usual suspects, your local newspaper want ads almost certainly include listings, for motorcycles. If not, get another newspaper. There are also motorcycle magazines that show-cases used bikes, plus your local motorcycle dealer, motorcycle club. Club members' motorcycles often represent some of the best-buy opportunities, because members aren't likely to sell a real headache bike to another member.
Those are merely your most conventional and time-honored avenues for purchasing a used bike. If you've got a computer with Internet access (very likely if you read this), brother, there's a whole world out there. The virtual universe is almost as vast as our own, so it's in your best interest to keep your search for a used motorcycle domestic. A chap in Chiang Mai in the North of Thailand might have just the motorcycle you want, but getting it home will be difficult.
The last advise is check the papers of the motorcycle, and not trust motorcycle dealers who promise the correct papers after 2 or 3 weeks after you paid. Also make sure that the numbers of the administration papers and the motorcycle match.