You don't need us to tell you that there's a huge secondary market for sports memorabilia - especially old sports memorabilia. A program from a notable baseball match or Super Bowl final can be worth hundreds of dollars, and sometimes more. A ball from a famous game, a used and certified shirt or jersey, a famous boxer's gloves from a well-known match; those are the kinds of things which we expect to attract a high price. We'd probably pass on the humble old poster, though. More fool us.
The highest price we know of for a poster is $61,000. It was a very specific poster, admittedly - an ultra-rare Cracker Jack poster that had been sitting in a barn since 1915, and was still in excellent condition. The owner of the barn had a friend who collected baseball cards, and tipped him off that it was probably valuable. Even he couldn't believe the auction price it reached, though. Do you have any in the attic?