White Ash is a species. I think northern just signifies that it is grown in a climate suitable to good wood structure for the bats. Slower growing is better for denser wood.
Never heard of Rock Hard Birch, but then I am mostly only familiar with Michigan trees.
I believe there is a Rock Maple, but never heard of the white hard part.
There are many hickorys, but "Hard and Heavy" must be indicating the particular selection of wood. Funny thing, I read hickory was given up because it was too heavy, but coming back because new drying methods were making it so they could get ligher weight.
If you look at his other Ebay listings or store, you can buy a single bat without warrant for IIRC maybe about $80 or so. Still, I would get a Louisille Slugger for less than half that.
He also sells billets for those who want to turn their own bats. You can't get them at Home Depot or your local lumber yard.