I would stay away from that Ebay deal... It says one license, which means the second install was probably used to upgrade the original user to 6 and they are selling off their hard copy of 5 to pay for the upgrade. Seen this done a million times. Technically, the hard copy becomes 'unlicensed' when the upgrade happens.
That said, I think any of the boxed products bypass the system now since the only active items are the CC editions. - Ron