Oniomaniac is the uncontrollable desire to buy things. Perhaps its being used to refer to Sarah Palin after she got almost 150 grand to splurge in not so Pro-America areas (like New York, Manhattan etc) of US.
Definition of Oniomania
Anyway, Oniomanic is somebody who is shopping dysfunctional. Perhaps they don’t know when to stop spending.
Oniomaniac is basically the incorrect spelling of this word. The correct would of course be Oniomania.