Author has written 7 stories for Final Fantasy VIII, Naruto, Harry Potter, Halo, Kingdom Hearts, and Beyond: Two Souls.
Note for Beyond: 2 Magical Cores - When shopping in diagon Alley I believe that a person can go and use normal funds just fine, especially at a Junk shop.
As for the pet store, well at that point the guy just wanted to get rid of the rabbit trying to burn his eyes out, knowing he can convert it later at Gringotts.
I imagine that all stores would take muggle currency, but at a higher price to compensate for exchange rates.
Note 2 - Harry was given a broken, useless wand in this story.