I did buy this off of ebay. I did look up the guy selling this item and he is a crew member of the show. This item listing was also mentioned on another crew members official twitter site. This was a gift from the cast to the crew commemorating season 6 and was given at Christmas time. You can see from the watch box there are small smears and you can also see some smearing of ink on the inside of the gift box.
I'm not that familiar with the show, but cool item! Considering the source and supporting info you have, I would say there's likely little reason to doubt it's authenticity.
Just a thought, in the event that you ever want to sell it, it might not be a bad idea to print out (or screen shot) the ebay auction page, along with any of that background/supporting info about the watch and just keep it on file.
I actually did that I copied all of the twitter activity so I will be able to print it out and I have all of the contact I had with the seller saved. I also have to ebay page listing saved so I can print the info out.
if you saved the twitter and ebay stuff then ur all set, 500 may be a bit much but it is nice having them on an item together. Personally I have never seen a full set on one item before that makes i really cool.