You could forgive Chris Moore of Redpoint Ventures for feeling a little pleased with himself today. Moore was the man who backed Right Media with a sizeable $12m solo investment back in 2005. He isn’t saying what kind of stake that money bought. Typically, though, investors in a first round of venture funding would look for around 20 per cent. If that is what Redpoint bought, that stake has just turned into $170m with Yahoo’s agreement to acquire the company.

So why the sudden flurry of deals in the online advertising business? When I caught up with Moore earlier today, he put it down to a fundamental change in the advertising landscape:

"What we’re seeing now is a shift in the online advertising space away from a very search-centric viewpoint. Search inventory only accounts for 3-5 per cent of aggregate page views on the Web. There has been an explosion of other inventory, around social media."

As Yahoo showed in its latest quarterly earnings, capitalising on that mass of inventory, much of it linked to email and other types of user-generated content, is not easy. One way to try to raise the value of this high-volume, low-priced ad space is to channel it through an online exchange like Right Media, which matches buyers and sellers of advertising space in the sort of open, transparent system familiar from the financial markets.

In this new world of online classified advertising, says Moore, "there is no silver bullet" that compares with the keyword-driven systems that have made the search business so efficient. Rather, he predicts, advertisers will have to tap into a range of different targetting mechanisms,  finding a blend that best suits their particular needs. Different ad networks and online salesforces will master different techniques for analysing and understanding the online audience: no one company will have a lock on all the data and methods, as happens in the search world.

If he’s right, then an exchange, or some other clearing mechanism, seems a necessary utility in this new online classified world. If you can’t maximise the value of all your inventory, why not turn it over to a market for someone else who can?

Of course, that does not by itself justify Yahoo paying such a big premium for Right Media (more than four times the implied value for the company when it first bought a stake last October.) Until now, Terry Semel has been put off by the risk of over-paying: he could have had Facebook last year for $1bn but balked at the price. Now, with Google gearing up to move into the online classified market, he has clearly decided to throw that earlier caution to the wind.