On my computer monitor the eagle definitely looks to be gray color regular M15 wartime Prussian wappen. I believe that the spike and spike base and split brads for the spike base are also gray metal, as is the rear back spine. The indoor incandescent lighting seems to cast a "yellow sort of glow or look" upon these mentioned fittings.
Is it possible to ask the seller to use a flash with their camera and re-shoot photos of this helmet?
Look at the inside of the M15 spike base knob from the interior close up of the inside of the helmet. It is gray color metal. The split brads are also steel.
I cannot tell for certain if the front visor trim and the 2-domed split brads at the end of the trim are brass, or if they are also wartime steel. There is such a "yellow" tint to the photos because no flash was used with the camera, and because old-fashioned incandescent light bulbs are being used nearby.
It is also possible that someone painted some of the fittings a gold color, such as the front visor trim piece.
Ask for some better photos, and ask them to use a flash with their camera when taking the photos.
Best Wishes,
Alan