It's illegal to reprint a poster and sell it. Here in Sweden it's legal to print it for private use in your own home if you get it directly from the distributor (i.e a company's press-site). But it's still illegal to sell it, and I'm guessing it's illegal everywhere you go.. except for some... less fancy places. lol.
As for genuine posters that are sold on ebay or whichever site, they too are illegal since they most certain don't have permission from the original distributor/company.
Anyway, my take is.. Do whatever you want, as long as your on the right side of the law. (IMG:style_emoticons/default/winking.gif