No doubt, you ve heard about the fierce price wars between Amazon.com (AMZN) and Wal-Mart (WMT) over best-selling books, and perhaps you took advantage of them to snag your copy of Going Rogue for just $9 (It s now $14.50 at Amazon, $14.49 at Wal-Mart.). But promotional stunts aside, who offers the best prices and selections among the online retail giants? This weekend I decided to find out by pitting Amazon, eBay (EBAY) and Wal-Mart against one another in a fast-paced, online shopping competition -- the Common Sense holiday challenge.

Of course, the nature of online shopping makes pinpointing the best deal a lot like hitting a moving target. As demand shifts or auctioned items are bid up, the best deal on an antique lunchbox may quickly become the worst. The prices listed here represent a survey of the landscape when we last checked on Tuesday afternoon.

Round One: Toys. Lego Knights Kingdom Royal Castle building set. Amazon offered a huge selection of the Lego Knights line, including the impressive castle and action figures on my shopping list. The featured item was available for $165.91 in a sale that qualified for free Amazon Super Saver Shipping, and other third-party vendors listed the item for as low as $149.98. EBay also offered a wide selection, including many slightly used offerings up for bid. The cheapest Knights Kingdom set including shipping offered on the buy it now option was $136.95. Wal-Mart didn t have the Knights Kingdom.

Wal-Mart did offer the Trolls Dark Fortress, which appears to be identical to the Trolls Mountain Fortress on Amazon and eBay. The Wal-Mart price was $94.88, although it was out of stock on line. At Amazon it was $99.90, with offerings as low as $91.49; on eBay it was $99.99. Score: 10 points each for Amazon and eBay, none for Wal-Mart.

Round two: Home appliances. Breville Ikon 610-watt blender. Some cooks swear by the looks and performance of Australia-based Breville appliances. Amazon offered this model for $149.99; on eBay it was $149.95, with reconditioned models from the manufacturer as low as $82.99. Again, I came up empty handed at Wal-Mart, which had no Breville offerings. But I did stumble over a Blendtec Connoisseur model at the eye-popping price of $649.95. I found the same model on eBay for $649.50 and on Amazon for $616.15. Amazon and eBay also offered a lower priced Blendtec model for $399. Score: 10 points for Amazon, 10 for eBay and none for Wal-Mart.

Round three: Movies and TV: Mad Men DVDs, all three seasons. OK, the Emmy-winning series about life at the Sterling Cooper ad agency is the gift I want for myself. On Amazon, the complete three-season set was $76.47 (that s $26.49 for season one, $17.99 for two, and $31.99 for the recently completed season three). But season three is a pre-order; it hasn t been released yet. Seasons one and two alone were $44.48. On eBay, seasons one and two were offered at $54.01 and there were plenty of used DVDs at lower prices. At Wal-Mart, season one was $26.32 and season two $17.86, or a two-set total of $44.18, plus free shipping (for the pair or individually). Score: Amazon 7, eBay 5, Wal-Mart 8.

The winner: Amazon, with a total score of 28. EBay was close behind at 25. Wal-Mart lagged far behind with 8. Amazon won through a combination of price and availability. When it was undercut on price, it was usually by a negligible amount. But eBay fared surprisingly well. It offered slightly used or reconditioned models at steep discounts, as well as many bidding opportunities. For price-conscious shoppers, the site offers a viable alternative to Amazon on most new items, as well as cheaper options. If my experiment is at all indicative, Wal-Mart needs a lot more than cheap bestsellers to challenge Amazon.

I d say the investment implications pretty much mirror the results. I own (and have recommended) Amazon and eBay shares, not Wal-Mart. For savvy investors, the question is price. Amazon shares have had a big run up, and boast a forward P/E ratio (71) in the stratosphere. Surely, they can be bought for a lower price after the holiday excitement has died down. In my view, eBay is an unheralded retail survivor, currently trading at a P/E of just 18.

Speaking of holiday gift-giving, this year marks the return of an especially heart-warming opportunity: giving appreciated stock to charity. I doubt that many investors were giving stock last December, not after the historic plunge in stock values. This year, we ve experienced an equally historic rebound, with stocks at or near their highs for the year just as the charitable gift season moves into high gear.

Getting a tax deduction for a gift of appreciated stock is surely one of the most generous aspects of the otherwise stingy Internal Revenue Code. If you ve held the stock at least a year, you get a deduction for the full amount of the stock gift, no matter what you paid for it. You pay no capital gains tax. The value of the tax benefit depends on your tax bracket and how much the stock has appreciated. But if both are high, the net cost to you will be far lower than the face value of the gift. And that doesn t count the many nonfinancial rewards of giving. Just about every nonprofit organization experienced a big drop-off in donations as a result of the financial crisis. They need our support now.