If you do a tour of all the expat bars and cafes, you will come across posters selling bikes. There also a couple of good websites where expats sell bikes and other stuff. You could ask the guys at this website 'The New Hanoian' which is run by an expat.
You will find it harder to find larger bikes in the south, but just do the same sort of thing - talk to all the expats, they also give you the best information on laws, roads, food and mechanics!
good luck! Just so you know, I brought a bike and sold a bike while I was in Vietnam for one month, its not hard at all, you just have to prepare your self for some down time and think about what you are selling and where. for example, I was riding a Honda SS50 from south to north. There is hardly any of these in the north so people jumped onto it quickly. If you are going to buy a Minsk then you need to ride from North to south. Any of these bikes can be sold - really easily in other countries such as Laos and Cambodia - to expats.
For more information on my adventures, please visit either of the following: