I'm not big on Buff's either but that's a pretty nice coin. One of the better toned Buff's I've seen.
That's a beauty you got there! Great purchase. I think something to keep in mind here is that all nickels are actually 75% COPPER and only 25% nickel. And we all know how reactive copper can be. I'm actually surprised we don't see more toned examples of nickels. I'm not a chemist so I don't know if the nickel in it tends to stabilize them but you don't see an abundance of toned nickels around. You know, I wouldn't worry about it. For that condition and that color, $175 was a steal. I'd sleep well over that purchase.
I do see a lot of nickels toned, but it is almost always a shade of light gold. Haven't seen one as radical as this before.
I see a lot of toned nickels. Checkout this thread on NGC that is chock full of toned nickels. Nickels for the Weekend
I think it's a beautiful Buffalo. That said, I'm not convinced that the toning is natural. It's important to understand that TPGs will slab a questionably toned coin if they deem it "market acceptable." This means they have a reason to believe that it could be NT and that the majority of the market will accept it as NT. They have rejected many NT coins because they wern't market acceptable and likely slabbed many AT coins that were. IMHO, I suspect that this could be AT...but it's a beautiful coin and I would love to have it in my collection.