Kevin,
I have purchased software on eBay a couple of times without issue. I recently purchased a version of Microsoft Office that was suppose to be the CDs and product key. Instead, I got a link to download the software and a product key without a genuine certificate from Microsoft. It did not feel right, so I had Microsoft confirm if the product key was genuine. Of course it was not. So I am in the middle of the dispute right now.
Here are the best three pieces of advice I can give you:
1. Only bid on items that are in the factory sealed package and ask a question to the buyer stating it is a genuine product key. If the seller responds and it is not, then you can get your money back.
2. Make sure you know if you are buying a full version or academic version. Academic version in some cases does not have all the features / functions.
3. If you make a purchase, do not use just a PayPal account. Use a credit card... not a debt card. This will give you extra protection.
Good luck,
Mark