normally i wouldn't get into a discussion like this, because ultimately, i don't care, but here's how i see the situation:
1) since you didn't create the flyer, you don't hold a copyright for it. that would be like me buying a kozik poster and suing someone who legitimately put part of the image on a t-shirt.
2) there was obviously more than one copy made. i would assume that you had your poster signed by the band quite a while before gigposters.com was around, correct? then where are the signatures on this poster?