I've bought a couple of large cardboard ads (advertised as period) featuring prewar baseball players, one for Draper and Maynard (a counterfeit of this Athletics panel:
http://www.robertedwardauctions.com/...2010/1718.html and one for Coca Cola (as seen here:
http://www.antiquehelper.com/item/313323). I actually bought two of the Coke ones, but now I'm pretty sure no such sign ever existed except as a recent fantasy piece. They actually look pretty good, but each was a case of my knowing that someone was getting suckered on the deal and just being incorrect about who it was.
I also paid more than I care to recall for a Sport Kings Jim Thorpe reprint and a Wayne Gretzky rookie back in about 2002.