Fred Wilson had a good post yesterday talking about the Fallacy of Zero Sum Game Thinking in internet marketplaces. The Zero Sum Theory suggests that as more sellers come onto a marketplace it hurts the early adopters. I’ve worked in internet marketplaces in 3 different industries -- real estate, e-commerce and now healthcare -- and I can tell you that this theory is a myth. I posted the following comment on Fred's blog:

The zero sum game theory is really just a misunderstanding of how good marketplaces drive traffic and acquire new users.

If most of Etsy's traffic came from them buying SEM or running TV ads, then yes, there is a fixed amount of traffic that sellers are competing for. But I'd bet that the vast majority of Etsy's new buyers come to them organically. That is, a buyer has a good experience on Etsy, then tells a friend, and that friend tells a friend, and that friend tells a friend, and on and on.

More sellers >> more good buying experiences >> more buyers.

The beautiful thing about marketplaces where traffic is driven by a quality buying experience (and word of mouth) is that instead of sellers competing with one another for traffic, they actually rely on one another for traffic.

I recommend checking out the original post. There's some great stuff in there on how, despite the controversy, Spike Lee raising money on Kickstarter actually increased funding for lesser known filmmakers. Great topic.