So I see the Field Dressing knife in Savage Spirits, are there any guidelines for selling hides (or tusks, poison glands, etc) anywhere that overlooking?

I use a narrative allocation of material called a Unit and affix a base price per unit of the Creature Resources (Hide, Meat, Venom Glands). Once you have the base price you can just use the regular rules for buying and selling.

Not really no. I'd probably just find something of equivalent purpose from a printed list, and give it a value similar to that, adjusted for location and demand. Hide= Some kind of common fabric, Meat=1 food ration, etc, and go from there.