As far as I know, the purchase counts in so far as BM rankings are concerned, and aren't retracted if it ends up unpaid or cancelled (the purchase history definitely doesn't change if a buyer doesn't pay or cancels, anyway).
I don't think there's any way for eBay to link the initial statistic to the eventual outcome and allow it to reflect in BM rankings, unless the transaction results in a defect, but that would affect the seller's overall standing rather than the particular listing (again, AFAIK).
