"As with all art, provenance is key to determining if a work is by the artist or a fake. However, with artists like Banksy, the early works were often sold without receipt because most people could not fathom that his works would sell at auction and in the private market at the value which they have. However, his career has only spanned for the last 20 years so finding provenance, even with his anonymity still in place, is not the most difficult thing. Because of the value of his work has become so extraordinary there are many groups which verify both his street art and his work which is sold in various reputable places (not on eBay.) You may think it is easy to fake Banksy's work but it has subtle nuances and takes great skill to pull off. Also because he got his start in England there are a few outfits which have had a lot of exposure to his work. Their livelihood depends on confirming both his public works and his pieces sold privately. Back in 2008 a few auction houses decided they better get stricter on provenance for his street works and set up an authentication board called Vermin." - Adriano