The above add lists an "original" Gold and Coal Fields of Alaska for $350.
A complete April 1898 issue with supplement averages $1016.66 (from my data collection since Dec 2008).
Here's a copy of the message I sent the seller:
I am sorry to say this is not a National Geographic issued "Gold and Coal Fields of Alaska" map.
It appears an authentic Department of the Interior production; this same agency provided the maps to the National Geographic Society to be issued as a supplement to its April 1898 issue.
However, all maps issued as a supplement have the following stamp in the upper right margin of the map:
NAT. GEOG. MAG.
VOL. IX, 1898, PL. 4.
Your map does not have this stamp and was therefore not part of the NGM collection.
Even the 1964 reprints include this stamp on the map with the additional word "REPRINT" in small caps centered below the above stamp in the upper right margin.
Good luck with your sale.