The serial number/license is really the only thing of value that you end up purchasing... the installation files can be downloaded freely, so any discs you purchase are basically only worth the plastic that they are made of. Your choice of purchasing off of ebay is the most questionable aspect of your plan.
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I should add that you can resolve any questions you might have regarding installation by downloading/installing trial versions. They will give you 30 days of free use and let you know ahead of buying whether or not you have installation issues. You do not have to uninstall them if you do purchase a license. Instead you can just use the serial number from the purchase to activate the trial to full use.
You can download the trial versions of the software thru the pages linked below.
Be sure to follow the steps outlined in the Note: Very Important Instructions section on the download pages at this site and have cookies enabled in your browser or else the download will not work properly.