Mark McGwire's 70th Home-Run Ball
Credit: Stan Honda / AFP / Getty Images

During the 1998 Major League Baseball season, back when home runs elicited more enthusiasm than suspicion, Mark McGwire hit 70 homers. At the time, it was massive news and a single-season record. Predictably, that 70th home-run ball went to auction, where it fetched $3 million in 1999. SCP Auctions did not handle the sale, but Imler would have loved the chance. He says baseball memorabilia (tainted by revelations of steroid use or not) tends to retain its value because of the sport's singular fan base.

"People are just very emotionally connected to the game of baseball," Imler says. "There's so many different categories of items, different things that can be collected from the game of baseball, and it's just a very, very broad and deep market."