From the original post a coin marked BU or brilliant uncirculated would need to have no indications of rub marks or wear, particularly at the high points of the coin which includes mostly around the top of the head on the obverse as well as the eagles head and feathers on the reverse.
There doesn't appear to be any obvious wear on the reverse, but here may be some wear on the obverse of the hair next to the cotton blossoms and around the cap fold. Even without any obvious wear there does appear to be rub marks around the eye and on the eagles breast, indicated by darker or discolored areas. This alone would knock the grade down from BU to AU even without the potential wear indicated. There may also be signs of a cleaning, but I think that would be very difficult to establish just with those pics.
The pictures, particularly the obverse, are a little blurry and don't always tell a complete story. You may want to take the coin in-hand and compare it to pictures of a low MS or Mid AU grade for that particular coin at the following webpage ... https://www.pcgs.com/coinfacts/coin/1878-s-1/images/7082