Walmart.com has broken through its own glass ceiling, selling high-priced platinum and diamond jewelry, cashmere sweaters and other goods designed to appeal more to the Tiffany crowd than to the bargain hunters who browse the company’s terrestrial stores. The question is why, and the answer is straightforward: Wal-Mart, the world’s biggest retailer, with 1.3 million workers and nearly $300 billion in annual revenue, is reaching out to more affluent shoppers. With disappointing sales of late and its stock price lagging, it has begun displaying more fashionable clothes and more upscale home furnishings in its stores. Over the last few weeks, it has added luxury items to its Web site as a test. The online store still caters overwhelmingly to buyers’ appetite for low-cost deals, from $18 six-packs of boxer shorts to $15 roller-skate kits, just as its stores do. But now, if you look hard, you can also find a Hyundai 60-inch plasma TV with a built-in digital tuner for $7,688, a gift basket of 1,000 fine chocolates for $248 and 100 percent cashmere scarves for $98.88.