Coins are like a high score for me, heh. I always hoard gold and platinum, just to see how much I can walk out of the dungeon with. No more than that, though, and in challenge games the gold sometimes gets left behind eventually... when you're toting like 50 stone worth of gold, it's time to offload. Most I actually tend to use the stuff on, beyond the store, is the occasional barter. If you've got enough cha (13 min, iirc), you can get some pretty nice buffs out of that.
Store itself... I pretty regularly nab some early game stuff -- all mages immediately buy leather armor and wear it. Do this, or bonebats will eat your brain (any coverage at all will negate bonebat stat damage -- yes, that means mage armor doesn't do jack against them.). Also small critters will trade in the small backpack for gnomish, if possible. My casters will do that if they're medium, too, since it offers a larger unique item cap and weight load, but smaller max item size (i.e. if you're not toting around weapons in it, you're better off with gnomish). Mages are also likely to buy a book or two -- the store tends to sell place and time and practical divinings, ferex, both of which have some fairly useful spells (call companions, detect secret door, etc.) that you want ASAP.
Beyond that, it's not uncommon to haul back for cure disease/poison, remove curse, or that ID wand if the dungeon isn't being helpful that day or (in the case of disease/poison/curse) I'm sufficiently desperate and full up on cash. Actual cha/diplo critters sometimes heads back up after finding transoil and plenty of gold, to pick up one of those ridiculous +5 doom weapons the shop's carrying or somethin'. Shop actually tends to have some killer kit in it, it's just by the time you've got the gold you've forgotten the shop exists, ha.
As for Knowledge (Magic)... I tend to forget it can do that, ha. Most non-scroll scribers just invest a bit in decipher script instead, coast on squeezing more out of ID scrolls. And the scroll scribers eventually end up able to scribe greater ID scrolls, at which point the identification game becomes rather superfluous, ha.