Just a bit of info for those who haven't tried, and aren't sure. I picked up a scarce poster several weeks ago, the back was covered in Blu-tack, which had been on a while. There was also some on the front which didn't show up on the photo they posted. I thought I'd better pick this poster up what ever the state, but the more I kept looking at it the more the Blu-tack annoyed me.
So I tried using Lighter fluid to get the Blu-tack off, and a blunt knife. It works a treat, and while it is still damp from the Fluid, or you can add more fluid after the Blu-tack has gone. I wiped the poster with a kitchen towel, and it has brought most if not all of the staining out caused by the Blu-tack.
I wish I'd taken some pics of before and after, but will do if I do another. I have not bid on a few posters due to them having this problem, but seeing how easy it is to rectify, I won't be put off in future....