I am sure I missed some of the contents because of the language barrier. I was impressed by the graph comparing what Voynich expected to gain by selling each book and what he actually got: his forecasts were quite accurate, though he consistently was slightly optimistic.
Well, the man had no formal training that would have set him up in the profession, he fell into it by accident.
He certainly was a good salesman and had an eye for what would sell. That doesn't mean he was an expert in the old books he was selling, just that he knew the market and had built up a good network of contacts.