Pricecharting and Gamevaluenow.com are what I use. That’s definitely a bit of a unicorn you have there and if you have more sealed stuff, it will also command much higher value.
One thing that’s happening now in game collecting is the people who collected comics, cards, sealed toys, and the like, are now looking for games. Sealed is their playground. They will pay and if you don’t get it graded, they will. That’s ok though because you can make a nice fee just because it’s sealed.
You should potentially list on http://www.retrogameboards.com I can vouch for you there if needed. There are other forums like that but you’d probably want to be around awhile before selling something so unique.
We’ve actually been talking about this a little on RGB. Folks there who have similarly weighty stuff (I do too… I have a sealed Castlevania: Aria of Sorrow for example…) say that Buy It Now on Ebay is legit and the way to go. Take a photo of the item in the packing box. Cover your ass in every way. Insure the shipping. Get delivery confirmation.
It is serious biz selling retro games now. Check your closets.