It seems to me that it could be made at some treaty negotiations (maybe at an agency) between the Sioux and Ponca. The indians in the back row look like Sioux, the men in the front row like Ponca. I assume the second person sitting from right (with a bearclaw necklace) is Standing Bear, the principal chief of the Ponca.

The photograph reminds me of pictures by Stanley J. Morrow, who often used blockhouses as background for his photo setting.

If you can, please compare the photo with illustrations by Morrow in “Frontier Photographer” by Hurt/Lass, especially the Standing Bear portrait (with bearclaw necklace and short hair) and portraits of Ponca halfbreeds, who look very similar to the men in the front row (including bandana turban).

Stanley J. Morrow moved to the Dakota frontier in 1868. For the next fifteen years he was active as a photographer, most of his photographs were produced in the 1870s. In my opinion the ebay photo could be made in this period.

Here are some of the said Morrow photos:

Standing Bear, Ponca Chief

Antone, Chief of the Ponca Half-breeds

Unknown Ponca Warrior

Big Breast Plate, Sioux

(From Wesley Hurt/William Lass: Frontier Photographer - Stanley J. Morrow Dakota Years, University of Nebraska Press and University of South Dakota, 1956)

— Dietmar Schulte-Möhring