Start searching online for sellers to get a feel for pricing. Be sure to try and match exactly, because variations can factor into value. If you're uncertain how to determine a variant, that information too should be online. As for selling, that's your choice, but Ebay is generally going to be the fastest, with the widest audience. As for shipping, you can buy poster tubes to ship which is an absolute must.
I would love to have one myself, but these original posters I've seen fetch up to $800+ and this one is huge, so that's going to factor in. Good luck, because actually the cost might hinder a quick sale if you want to get maximum value. You'll need to be patient, most of the time, to get the most money