Greetings Paul,
This growth is commonly referred to as a burl - abnormal tissue growth induced by dormant buds (although the exact cause of burls is still debatable and unknown). Some burls create highly figured wood and are prized by woodworkers.
Burls are relatively common and many trees with existing burls are still considered healthy. Therefore, I wouldn't go hacking it off with a saw simply to sell it to a woodworker.
