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I've bought software from ebay and if you're careful you're fine. if the deal looks too good to be true, it usually is. but the item you linked (which will work with your macbook, by the way) is being sold by a power-seller. they have more than 10,000 items of feedback, with 99.7% of them being positive. so, you should be fine with purchasing that copy of windows from that seller. go for it!
 
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