What motivated Marco Polo to share his trade knowledge?

It is now widely believed among historians that Marco Polo never made his journey, that he invented his tales based on stories he picked up from other travelers he met closer to home, stories retold time and time again along the Silk Road.
There are too many inconsistencies, outright errors, and glaring omissions in the stories (and too many versions of them out there).
http://www.aliasoft.com/themes/polo.html
http://website.lineone.net/~mcrouch/marcopolo/liar.htm
http://usatoday30.usatoday.com/life/enter/books/leb282.htm

If he never made his journey, there was no first hand knowledge to spread and all he did was get rich(er) and more (in)famous off of telling his story, which can't have hurt his business enterprise.

Polo was already fantastically wealthy, so he may have been ambivalent about money.

The operant issue was that at the time he wrote the book he was in a prison in Genoa. Writing the book had the advantage of improving his reputation with the Genoese and thereby helping him get out of prison, which he eventually did.

As far as the benefits of the knowledge go, I think it is a pretty abstract question. The Venetians controlled the East Meditteranean trade with the Arabs and Polo was able to make his trip during a window of time when relations between them were particularly good. As the years wore on, the Ottomans became more powerful and the kind of trip Polo made became increasingly impossible.

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Another important thing to know is that the book did not come out of the blue. Marco Polo had been telling his stories long before. There was a steady parade of noble Genoese coming to visit Polo in prison to hear his tales. It was a natural thing for him to publish.

We can safely assume that Polo wasn't motivated by greed or monetary compensation when he undertook his voyage but rather by spirit of adventure and curiosity.

There are several reasons why money was not an issue when he decided to propagate his knowledge, but first and foremost it wasn't an issue because his family was already well-off else he wouldn't have been able to travel through Asia (as his uncle and father did too).

Furthermore I doubt that the book would have brought him any advantage, as he would not have been able to capitalize much on his knowledge. Apart from the fact, that you would have bind a huge amount of capital over long periods of time (land travel on foot from china can take several years), establishing direct trade cutting off all intermediary trade also brings huge risks. Venetians didn't need that, because the products they traded with were mostly luxury products like silk, which they could get enough of a profit in by importing them (already with high prices but without the risk and the voyage) from Arab and Crimean intermediaries. Cutting them off would also have deteriorated their relations...which brings me to the next point:

Direct trade between china and Venice was a political and logistical impossibility at the time. Travel through land required permission and support of a vast number of lands and rulers, which often weren't stable. They would have to establish permanent contact with every one, send emissaries and probably bribe more than it was worth, and it still would be too long a way to effectively guard, control and secure. Travel through land also requires large, bulky, exposed and very expensive convoys and caravans, for what was traditionally done by short range intermediary merchants without a supply problem. The other option would be by sea, but the Venetian Fleet wasn't equipped with the knowledge, supply ports and logistics and the ships to sail around Africa - mind you, the Suez canal wouldn't be built for another 5 centuries. The countries you cut off the trade were countries which based their wealth on spice and silk trade, so they wouldn't be happy either - and you need to pass through their land.

Another important point is, that Marco Polo was in prison while he dictated the story. Being a Venetian in a Genoese prison generally wasn't a good sign for your personal commercial enterprises. He eventually got out, but I don't think he ever regretted it.

Because last but not least...probably what counted more for Marco was fame, or a sense of duty to pass on his knowledge or some other intangible motivation. Money he had enough.