the way you should approach any high priced item is that on the whole is this an item with good longevitiy relative to price... you'll most likely be able to wear Cup's tomorrow, next month, and 5 years from now without looking out of place in my opinion. Think of it as an investment.
but fuck who cares about my opinion... i think ebay is probably not a good bet
here's a guide I came across reviews.ebay.com/Spotting-Fake-Prada-Shoes_W0QQugidZ10000000001093756
and gives a list of legit online dealers