You have no obligation--not in the eyes of Etiquette and not in the eyes of Morality (my opinion)--to try to be "fair" to everyone who might ever be interested in these posters.
Don't make your life nuts.
Just put up a post that says, "I have these posters. If you want one, you have to make it easy for me to send it to you. Drop me an email."
and then just send them to whoever proposes the easiest way to send them. Your call--you are completely allowed to be totally capricious.
You can send one to Audrey bcs you have fond memories of a former roommate named Audrey; you can send one to the guy in Japan bcs he offers to send you something cool in return, and you figure even if it doesn't, you like the idea; you can give one to Pete because he lives near you and will swing by and pick it up.
They're yours, and there is no global requirement to do this in any way other than the way that appeals to you.
Of course, if you like the fun of setting up a contest of some sort, etc., have a good time!