Pattaya Super Bikes is where you'll want to go if you're looking for used / previously owned motorcycles. They're in nice shape, but, not spectacularly cheap.
They have a Honda Shadow 400 cc for 199,000 Baht. (6600.00 u.s.)
They have a Honda CB400 Super 4. ( it was marketed for Asia only)
Nice bike, the price, 175.000 Baht. (5800.00 U.S.)
A Yamaha for 165,000 Baht.(5500.00 USD) Nice Clean Bikes and they all have green books, which you'll need for no hassles down the road. (to buy and sell/ and to keep the police from taking your bike away from you. You MUST have an up to date green book.
You'll find Pattaya Super Bikes off Sukhumvit, down Soi Siam Country Road. (just north of (Klang), Pattaya Central off Sukhumvit)

If you want to buy a scooter there is a Yamaha, or Honda shop every few miles. Once in a while when you stop in one, there will be a larger bike. But, most only carry scooters. A scooter page, coming soon.