I've seen Back to the Future at least a hundred times with my brothers and sister, which is why I'm excited about sinking myself in debt to get Marty McFly's resizing jacket, his Nike sneakers and Biff's porn mag.
This is the real thing, not a fake. It will cost you from $25,000 to $50,000 at auction shop Profiles In History. They also have the original Nike self-lacing shoes—which look kind of crappy for $15,000.
The French porn magazine that Biff uses as a Sports Almanac decoy. According to the auction site, the 28-page magazine "has actual 1950s nude photo pages, repeated several times."
You can also buy the 1885 prop map of Hill Valley and the original Outatime plate