Practically no artists are really discovered posthumously. The guy who wrote a confederacy of dunces was discovered posthumously. He wrote one published novel in his life and it did shit sales.

Herman Melville borders on being a posthumous discovery. He was panned by critics but people kept printing his new books. They might have sold like shit and gone out of print, but publishers kept publishing his new novels.

A big name in the literary world for like the past 15 years has been Richard Yates. All of Richard Yates's novels had gone out of print in his lifetime and were out of print then rereleased for small sales and this cycle continued. However, Revolutionary Road was made into a movie in I think it was 2010. Really, he was a known artist. Writers highly valued his work. It's just that there wasn't an audience for it.

I'm pretty sure that's how it was with van gogh. I completely believe that he died penniless. But that does not mean he was unknown. If he was unknown, his painting might have sold at the kind of galleries you see in shopping malls or something and that would have been it and he would have been completely forgotten. He would have had to be "recognized" and put in a notable gallery in order to receive the kind of fame he has now.