Where to Buy a Decent Used Motorbike in HCMC?

You can try checking out the used bike dealerships that run along Phan Dang Luu Street in Phu Nhuan, from about Nguyen Van Dau continue West (direction of the airport) right on until the first section of Hoang Van Thu. Numerous dealers dotted along this stretch. As always you will get a higher price from a dealer than a private seller, but at least it will save you driving all over town meeting private sellers from Craigslist. If you are after a dodgy dirty backpacker Minsk then suggest you try the Pham Ngu Lao area.

Agree with NicoM.
Otherwise you can go To Hieu street Q. Tan Phu or Da Nam street Q.8 to see a various make and model motorbikes  if you have a good knowledge in checking the same.
Better do not buy a second hand motorbike more than 2 years old.

You can buy a brand new Honda Wave for a 1000 bucks.

The used bike dealerships are really sketchy: most of them just fix up the bikes so that they seem good enough for a few weeks; the low quality parts they use wear out quickly and then it's one problem after another. Not to mention the astronomical prices they quote.

A friend of mine bought a nice looking bike, thought he got a bargain. The very same day it stopped running. Back at the store he gets the classic hand waggle: the money has exchanged hands and they suddenly forgot how to speak English. A month later and after forking out a lot of cash at an expat mechanic's shop it finally runs well.

Mua Ban is hit or miss, since most Vietnamese don't really take care of their motorbikes. They'll shell out for an expensive scooter but don't bother with regular maintenance. When something does break, it's off to the cheap corner bike store for a temporary fix. A few years of that and they invest a bit of money to fix it up for sale, lather, rinse, repeat. The engine gets royally screwed up in the process, having gone through the hands of dozens of amateur mechanics who each have their own set of notions on how to repair a motorbike.

My advice is whether you're buying direct or from a shop, bring the bike to a trusted mechanic (ask a friend) or have him come with you to the store, it's the only way to minimize the risk of getting a lemon.