I think one of the most challenging parts of building a "marketplace," which for discussion purposes I'll simply say is the same as a community or forum as long as one party has something the other needs, is keeping both sides of the table balanced. For ebay, these parties are buyers and sellers. For Trulia Voices, these parties are home buyers/sellers and real estate professionals/local residents.

Having Trulia Voices w/in a larger real estate site - Trulia.com - helped resolve some of this need at launch; we were able to leverage current visitors and partners to drive awareness and early usage of the product. Questions initially came from onsite promotion and answers came from marketing outreach to the industry. [For products not imbedded w/in larger sites, partnerships with other services/communities seem to be the great substitute (as some of my friends are beginning to discover in their own product development efforts).]

Keeping both sides of the table in equal proportion is a challenge that extends into the lifetime of the product. There are constant tweaks that need to be made in order to assure each side is contributing at the level needed. As one side scales but, another set of changes must be made to assure the other side matches.

This challenge is what makes online marketplace development exciting - your creative hat goes and you hash through the ideas to find the answers. And, each time, the answer is very different.

After 3+ months of development, one too many lessons in how I can improve my communication skills (I hope this is an age thing, ie skill acquired over time), and many uses of my new favorite phrase "i'm thinking" when caught staring into the blue or traversing the office in deep thought, Trulia Voices has FINALLY launched (for the full scoop see here). I left the office at 4AM (engineering at 6-7AM!) and came in around 10:30AM bouncing around between office cubes with an excited "we're live, we're live, we're live... guys we're alive!"

Personally I feel that if you can not acknowledge your weaknesses and focus on trying to improve them, then you can never achieve your full potential. And, I certainly learned a handful of lessons in this launch. Some of the biggest ones were:

1. If you can't explain why something won't work in a way that makes sense to others, then shut the hell up (advice straight from dad, and it saved my rear one too many times)
2. don't tell higher-ups, "no, you're wrong". hmmm... :)
3. instead of telling people what you think the best way to execute is, present the problem and ask for a solution. Maybe this one is obvious but from my eyes i think it's a lot harder to learn how to do this one well across a big organization when everyone on a team is not always involved in a product at every stage.
4. If you think you're right and everyone else disagrees, stand up and say it! This is an old one, but i always relearn it.
5. Do lots of yoga

And I'm sure i'm missing many more but what a start... If I knew better i prob.would not be posting this but personally, i don't think these issues are all unique to me. I think many my age encounter their own set of lessons and communication issues. Then again, maybe I'm wrong...