Trading card leader Topps Inc. may be ripe for a sale, after seven years as a portfolio company of Madison Dearborn and Michael Eisner's private equity arm. But a lot has changed in the trading card model since private equity spent $385 million to buy the asset back in 2007. The trading card business used to be based on a certain scarcity value: that Mike Schmidt rookie card was worth what it was worth - as much as $900 - because a limited number of those cards were manufactured. Now the business has to consider a digital strategy. Topps' push into digital games played on mobile platforms sets it up to unlock some value.