Was your friend's poster 'better' that the one you received i.e. more signatures on it? My completely baseless speculation is that they sent your friend's poster to the person whose poster you now have, and that person kept it. SM's fault, but very difficult for them to fix.
As for the trusting SM with personal items bit, it seems like they get 99.whatever% of items back safely (made up statistics alert). Personally I don't think I'd risk it with SM or anybody else if it was one of my posters that I'd truly be upset about losing.