As a time-constrained mother of four young children, I visit Wal-Mart about twice a week to buy diapers, school supplies or clothes. Inevitably, when I finish loading my mini-van, I find my short trip for one item turned into 10-15 items bought at well over $100. Why does this shopping magic happen at Wal-Mart but not at eBay? At eBay, I find the "one" thing I'm looking for and then I'm done. Simply put, the difference between shopping at eBay and Wal-mart is serendipity.

Serendipity is discovering something you want by accident. As hard as I try, I cannot move from one spot to another inside a Wal-mart store without passing about 3,000 items. When one catches my eye, if the price, timing and budget are right, I buy. That's the secret: Wal-mart taps into my pocketbook by tempting me with products to buy as I walk around.

So why is my eBay shopping experience so different? One reason is that over 75% of all eBay purchases start with the search function rather than browsing. eBay search is clearly a powerful tool, but it limits what a buyer might serendipitously see when shopping at eBay. And seeing is crucial to buying.

Don't think Wal-mart is lucky because buyers walk from point to point in their stores. It's by design that the diaper section is completely opposite the toy section. Amazon has also designed serendipitous selling into their business model. Look at the bottom of Amazon's item pages and you'll see what "Other shoppers who viewed this item bought..." Who can resist looking? How about Amazon's free shipping offer? With an average item price just under $25, choosing just one extra item to qualify for free shipping nearly doubles Amazon's revenue per transaction - serendipity abounds!

Great retailers always design maximum serendipity into their business. At Wal-mart, that means showing off as many products as possible using strategic placement. At Amazon it means showing highly targeted products.

Unfortunately, the current eBay cross-selling tools pale next to the clever designs of Wal-mart and Amazon. Once eBay CEO Meg Whitman exhausts international and category expansion, she will bring all of eBay's impressive energies to bear on designing-in great serendipity so that a one-item purchase expands into 2 or 3 or 4 items.

In the meantime, if we as sellers don't figure out how to do this, the folks down the street at Wal-mart and Amazon will end up picking all the available dollars right out of our buyers' pockets.