Although I am no expert there are several things, which in part have already been mentioned above, that worry me.
First the price, which apears indeed ridiculously low. For example, a similar helmet (of excellent quality) is offered by Weitze for nearly 20.000 euro's....
https://www.weitze.net/detail/14/Preussen_Helm_fuer_Offiziere_im_Regiment_Garde_du_Corps_bzw_Garde_Kuerassier_Regiment__108714.html
Second, the parade eagle appears to be in an strange upright posistion (see for a comparison the helmet offered by Weitze and also similar helmets depicted in various reference books, viz. page 71 of Kubes's study of German of helmets and uniforms).
Third, no pictures of the interior. No pictures of the interior are shown on ebay, which is suspicious because it may point to, for example, a repro liner etc. However, you obtained some pictures of the helmets interior directly from the seller. As far as I can see this may be ok, although the bolt in the middle to attach the eagle on the surface of the helmet seems of a different type than the one on the helmet offered by Weitze. This may be explainable but I am no expert on these helmets. However, I think is suspicious.
These are just 3 points that are of notable concern in my opinion.
Best regards,
Edwin