In case you weren't already aware, there are several good sources on these unusual autos, two of the best being "Webley & Scott Automatic Pistols" by Gordon Bruce and "Howdah to High Power: A Century of British Breechloading Service Pistols (1867-1967)" by Robert J. Maze. [Since you quote the correct figure of 520 delivered to the Royal Australian Navy (RAN), I assume you might already be aware of one or both of those.]
Most authorities report that the Mark IN naval version (only those for the RAN were marked "Mark I"; those delivered to the Royal Navy and other associated Commonwealth navies were marked "Mark IN") remained in use (either reserve or substitute standard) by those navies through WWII, while Maze states that the Mark IN was last issued as a substitute standard in the Royal Navy during the 1956 Suez Crisis.
If you sell, don't let it go too cheaply. The M1913 is an uncommon bird, and in a modified form with adjustable rear sight - known officially as the Mark I No. 2 - intended for land use by the Royal Horse Artillery, it is quite rare. (Many of these 439 No. 2s ended up being used in WWI by the new Royal Flying Corps and Royal Naval Air Service.) The 520 Mark I that were delivered to the RAN would also seem to fall into a very scarce category. Mine (SN 279x; about 85%) is another one of those delivered to the RAN in 1913. In addition, I also have one of the super-rare 1907 naval pattern holsters (made with seams secured with rivets instead of the usual stitching that was too susceptible to a saltwater atmosphere) with shoulder strap, originally intended for Webley revolvers, that was modified (pattern 1907/1915) by putting a second hole in the flap to accomodate the different grip position of the Webley Mark IN autos. The only one of those holsters I've seen on the Internet or elsewhere in the past several years carried an asking price of almost $500.
Hope this info is helpful. Good luck if you sell the RAN Mark I (but I'd think long and hard about selling it, if I were you).