Your best selling shot? What size do you sell it, and for how much? Framed, or unframed?
I think if its selling, people must like it, so I'd leave it alone. It's hard to say what, if anything would make it better without seeing first the native file. As for this shot, I think it's very nice. It's balanced, it has good color and the buffalo shows signs of life. I would like to see one as a landscape. I think that while this shot is balanced, the sky and most of the trees is just "extra" stuff. The top 1/3 of the shot could be removed and you would not lose much. I think if I were going to try to show the animal in a big, cold, lonely place, I'd try a landscape with lots more room about the front and back. It may not work, but then again it may.
It's a nice shot as it is, it screams cold yellowstone, so if that's what youre after, you did it.
It's such a small file and it's been compressed so m much it's hard for me to tell if the head is in focus. If it is, great, if it's not, then I dont know how you cold impove this shot. I looks to me like your focus hit the body and not the head, or the grass just in front of the animal, but for some reason it looks like the head is a little out of focus.
If it's in focus, I'd leave it alone, it's a nice shot. You askd for comments, so I just thew out some picky stuff for you to consider.