I would expect it to be disclosed and I would if I was the seller, but it's also fairly minor. Based on the wood putty on the corner it looks like it happened a while ago. An honest seller might forget about it since it is out of sight when the machine is set up.
How big of a deal is it for you? How much less do you value the machine because of this?
Also, how was the machine described/advertised?
EDIT: ^^^ he beat me to it