AFTER A SLUGGISH few years, sales are finally stacking up for the toy industry. Toy sales in the United States are forecast to be $19.9 billion in 2013, in comparison with sales between $16 billion and $17 billion in recent years, according to market research firm Euromonitor. However, the lion’s share of these sales went to online behemoth Amazon, whose marketplace offering leverages thousands of third-party merchants—and even big box stores felt the heat . . .