Greed might be one theme of Zora Neale Hurston's "The Gilded Six-Bits" -- to me, 'reconciliation' or 'trust and betrayal' might be a more precisely worded theme -- but it's not the plot. At least not according to the definition of "plot" that I know.
Plot can be defined as the series of events in a story that, taken together, achieve the overall effect or theme of the story. The plot summary thus might begin: A newly wedded couple lives a happy, playful life until a sophisticated stranger comes to town. ... A good plot summary should leave out small details (e.g. the ice cream parlor and other settings might be unimportant) but should mention important plot devices, such as the gilded coin.
The coin is both a plot device and a symbol, I think. The coin seems to haunt Missie May, maybe like her bad conscience, just as it seems to be Joe's way of showing he's been hurt and hasn't forgotten his wife's transgression without having to say a word.
I hope that these comments are helpful!