I would like to caution you about purchasing a puppy from a classified ad. If you consider doing so please become very well versed with the screening questions that Pat sent a link to at, http://www.basenji.org/learn. Also, here is an article describing the qualities of a responsible breeder, http://kineticbasenjis.tripod.com/Information/Buying_a_Basenji.html
If you are buying a puppy from a breeder, make sure it is a responsible breeder who is breeding with the best interest of the breed in mind. Also, a responsible breeder makes sure that puppies are health tested decreasing the likelihood that they will be afflicted with a genetically inherited disease, that they are well socialized so that they will be a stable well adjusted companion for you, and that they are there for you no matter what your question or if you need to return the dog.
It is sad to hear about so many basenji puppies being surrendered to shelters but they end up there because irresponsible breeders who are hoping to make a profit or wanted to show the kids the miracle of life or just thought that "fluffy was the perfect dog and wanted another just like her," didn't take responsibility for them once they were here. If they make a profit on the litter even if they surrender half the litter to a shelter, there is a good likelihood that they will breed again continuing to add to the shelter crowding problem/