Mr. Google informs me that there are no special terms for male or female beavers, although juveniles are called "kits". He further states that, unusually among mammals, male beavers are typically smaller than females, so a prime beaver pelt would probably be from what might be but isn't called a "doe" beaver.

Finally, off my own bat, "buck" could refer to a deer CARCASE rather than its hide.   Hide for leather would have been much less valuable than pelts for luxury goods and so would be less likely to be reckoned a medium of exchange. But fresh venison was much in demand at trading posts and was typically purchased from local Natives.