In the film The Hobbit: The Battle of the Five Armies, who plays the role of the Auctioneer who is in charge of disposing of Bilbo's house and belonging?
Was it Ian McNiece from Doc Martin or someone else?
Based on what I could find I'd say it's probably Mervyn (Merv) Smith.
On the IMDb full cast & crew page Mervyn (Merv) Smith is credited as Tosser Grub (although this list is awaiting verification). Peter Jackson's The Hobbit Wiki states that Merv Smith portrays Tosser Grubb in The Battle of Five Armies as well as the extended edition of An Unexpected Journey. If this is the same character it would mean that he turned grey over the course of Bilbo's journey.
I checked the credits for An Unexpected Journey (see first image below, bottom right) where Tosser Grubb is listed as portrayed by Mervyn Smith, the same goes for the The Battle of Five Armies credits (second image).