I agree with everything Gem said. The only thing I can point out in addition is the mane and tail, it somehow feels like there is a discrepancy between the body prep style and the long hairs. Maybe the best way to explain this is that it feels like the mane and tail prep was done for a different piece of art than the body prep. Especially in the second and fifth examples.
Sadly manes and tails are my weakest points, so I don't feel qualified to give tips on how to fix this. Your shadows within the mane and tail itself are already very nice. Maybe the problem is that I see less detail in them than the body prep? There is the gradient of the base color and then one lighter, thinner line following along the flow of each. Maybe if there were more of these thinner lines inside, both light and dark, it would help? But then again, I don't know how much of the mane and tail prep you do is a choice of style, so I don't want to put my nose to where it doesn't belong x)
I was eyeing a few of your auctions too - sadly, didn't have a horse for either. And while I don't think 200k+ is an unreasonable asking price for your pieces - quite the contrary -, the high SB deterred me as well. I can't quite explain why, it's sort of instinctual almost. So I agree with Gem on the question of a premade shop as well.