You didn't mention if it's a cut in the poster or a 1/2 inch piece gone from the poster. The thing with new rolled posters is condition is everything. Any little ding or crease will decrease the value to some extent. A hole in the poster is impossible to hide, even if you use scotch tape on the reverse side, which is a big no-no. If the poster is double sided any tape you use on it will be seen. Personally I wouldn't try to sell it with a defect like that unless you tell the buyer it has a hole in it and give him a deep discount on the price. You may have to use it as wrapping paper.