You should not purchase Autodesk products from eBay. They are most likely, pirated, stolen, or fakes. It is not legal for anyone to sell softare on eBay and the Autodesk EULA says that licenses cannot be sold or transferred.
From the EULA:
3.2.3 Transfers. You may not distribute, rent, loan, lease, sell, sublicense, or otherwise transfer all or any portion of the Software or User Documentation, or any rights granted in this Agreement, to any other person without the prior written consent of Autodesk.
3.2.8 Export. You may not export the Software or User Documentation in violation of this Agreement, U.S. or other applicable export control laws.
3.2.9 Use Outside of Territory. You may not use the Software, including without limitation a Network Version, or User Documentation outside of the country in which you acquired the Software.
Furthermore, eBay has a VeRO policy for Authorized Intellectual Property Rights Owners. If you believe that an auction violates VeRO policy, please see the following site:
http://pages.ebay.com/help/confidence/vero-rights-owner.html
Autodesk Piracy Prevention webpage:
http://usa.autodesk.com/adsk/servlet/item?siteID=123112&id=294744
Report any auctions to eBay and Autodesk. Only purchase Autodesk software from your Authorized Reseller.