Before we answer, a note about trolls .... If you're a fan of trolls, like the Dear Rich Staff, this has been a tough week. First, there was the Charlie Sheen rants, then the FTC announced it was looking into patent trolls, and then to top it off, the Supreme Court agreed to hear a case that could cause the Danish trolls to get dropped back into the public domain (now, that would be so sad). Please take a moment to reflect on all the good things trolls have done for us and if you have time, go ahead and file an amicus in support of our little buddies.
Right, you had a question .... If the rubber stamp is protected under copyright law -- most are not, by the way -- you'll need permission from the copyright owner if you want to reproduce it in your trademark. The reason most stamps are not protected is that they consist of: (1) imagery published before 1923, (2) simple shapes or "generic" imagery, or (3) short phrases or single words. So like so many confusing things in copyright law, some rubber stamps meet the standard and some don't. You'll need to make your own analysis and our favorite choice for guidance would be Steve Fishman's Public Domain book (it walks you through the analysis and provides checklists).
What about licenses. Some companies seek to prevent you from using otherwise unprotectible stamp imagery under the guise of a license (sometimes mistakenly referred to as an angel policy or cottage license). These agreements are often not enforceable because the user is not presented with the license before purchasing. That's essential in order for you to agree to the terms or conditions (referred to commonly as a clickwrap agreement).