My understanding is that Vincent Van Gogh was a poor painter for his entire life. If he only sold one of his paintings during his life, then where were his many paintings stored?
Vincent's brother Theo was a successful art dealer and he stored Vincent's paintings. When Vincent died Theo was in the procession of 200 of his brother's paintings in his Paris home and about 70 more that Vincent painted during the last few months of his life. Unfortunately, Theo died only two months after Vincent died. The paintings were left to Theo's wife Johanna. Johanna is largely responsible for Vincent Van Gogh's posthumous fame as a painter.
Source: http://www.amazon.com/Van-Gogh-Auvers-Last-Days/dp/1580933017
It wasn't until after Vincent Van Gogh's death that collectors really started taking his work seriously. At the time of his death, his sister-in-law Johanna Van Gogh (married to Theodore Van Gogh, who is Vincent's brother) took it upon herself to do something more with all the paintings Vincent had left behind - and yeah, there were a lot.
She wasn't an art dealer or collector by trade, but coming from a wealthy family made it a lot easier to start making connections with other art dealers and collectors -- and Theo himself was an art dealer, so it kind of made sense to go down that route.
For Johanna and Theo, it wasn't about getting rich quick off of the death of their family member -- Theo was very respectful of art overall and treated Vincent's as especially precious -- he wasn't the type to sell a piece of his at a yard sale. When they did sell, they went to good homes and for good money, and important collections were often kept together.
To piggyback, why is van gogh considered a genius? His work seems crude and simple to me.
His Brother and Sister-in-law had a gallery. They had both been big supporters of his work, so they kept the collection together. From what I understand it was mainly Vincent's Sister-in-law making people look at his art, that made him famous. She kept the collection together for a very long time.
My source is the Stuff you missed in History class, they go over this part in the last 6 minutes. I believe they list their sources if you want something more historic.
Vincent van gogh biography, very impressive.. but turkish. http://youtu.be/l8ieeDINk6M