That's actually an interesting enough theory that I'd love to test.
My guess is that the site is generally mediocre enough that it would be hard to get more people to open their wallets except for select listing types.
If they charged to view the listings for example, I'd wager that very few people would bother and they'd lose their network effect.
My guess is that they're getting about as much money as they can without offending too many people as preservation of their user network is highest priority, as it is what allows them to keep charging money for the few things they charge for currently.
