It's wierd. I want tanners to buy Spined, horned and barbed leather so i did what you should do and added this to the sb buy info.
this.Add( new GenericBuyInfo( typeof( SpinedLeather ), 6, 20, 0x1081, 0x8ac ) );
this.Add( new GenericBuyInfo( typeof( HornedLeather ), 6, 20, 0x1081, 0x845 ) )...