A:This is a difficult question, I'm not sure the information is available.
This report (pdf) says that "In total, the global public auction Fine Art market generated a total revenue of $ 6.4 billion", but I don't know what's the total size of the market, and what's the share of the auctions or the online sales in it. The data about auctions is certainly the easiest to find; whether or not anyone collects data about sales in galleries, private sales and online sales - I don't know.
