How much is Yahoo's stake in Alibaba Group worth? Nothing if Yahoo can't sell it.

Yahoo is considering selling part of its 40% stake in China's e-commerce giant back to Alibaba, as it considers options from its own potential suitors. This time, the talk values Yahoo's stake at around $12 billion, slightly less than the $14 billion value Yahoo put on it in its third-quarter earnings. Not bad, considering Yahoo paid just $1 billion back in 2005.

If Yahoo had the luxury, it would probably hold its stake for a few more years. China's e-commerce is near the beginning of a structural growth story, with rising consumption and a shift toward more online spending. Alibaba -- through its Taobao unit -- is the dominant presence. Anyone selling his stake in Amazon 10 years ago would be kicking himself. Yahoo doesn't want to make the same mistake.

But there is also pressure coming from the other direction. Taobao's growth is set to slow from super-charged to merely very impressive as the base for comparison grows and rivals jostle for market share. Analysts at RedTech Advisors, a China-based technology research firm, say a doubling in Taobao's sales revenue in 2010 is unlikely to be matched in 2011. With investors looking at growth as the basis of valuations, multiples now are probably as good as they will ever be.

More important, Yahoo's stake isn't worth anything unless it can sell it, and apart from Alibaba itself, there aren't many buyers out there. A clause gives Alibaba right of first offer if Yahoo ever decides to sell. Alibaba Chief Executive Jack Ma's famously difficult relations with Yahoo management reduce the incentive for any third party to wade in.

Holding on until Mr. Ma launches an initial public offering for Taobao would be one way to unlock the value. But Alibaba is under no pressure to take its most valuable asset to market, and might even refuse to do so until Yahoo has reduced its stake. Mr. Ma's move to shift online payment provider Alipay out from under Yahoo demonstrates that there are risks to trying to wait it out.

In an ideal world, Yahoo would hold its stake and ride the rising wave of China's e-commerce. In the real world, that might not be possible. A deal that gives a reasonable value to Yahoo's stake, and allows it to hold a portion to benefit from future growth, looks like the best it can hope for.