-An obsessive or uncontrollable urge to buy things.
-An abnormal impulse for buying things.
From Greek ōnios 'for sale', from ōnos 'price, purchase’ + Late Latin mania "insanity, madness," from Greek mania "madness, frenzy; enthusiasm.
Used in a sentence:
“When it comes to the buying of books, there is nothing wrong with a bit of oniomania.”
(from The Grandiloquent Word of the Day FB page)