What is the pros and cons of a marketplace website like houzz.com, zillow.com and trulia.com where users send mail to professional without logging in?

You could try a "widget" on the lawyer's site which facilitates getting generic questions answered for free. The idea being that in each practice area, there might be a handful of questions that they get asked frequently, and would commit to answering one-time. It could be used to qualify the web visitor (always a good thing) while satisfying the visitor by providing them an answer. Of course, the challenge here is that most lawyers might only be comfortable providing such watered-down generic advice, that the answers themselves wouldn't be very useful. But this way, you could provide value to lawyers somewhat comfortable with online discourse, while building up content. With enough lawyers and content, you could then expand the service to build towards your larger vision. But as John has mentioned, many entrepreneurs have and are actively trying to win with this type of idea and have often struggled. CaseText is a recent YC grad that is doing some interesting work in this area. Happy to talk through your product implementation.

It's very important. (first, read this article by Josh Breinlinger - http://acrowdedspace.com/post/47647912203/a-critical-but-ignored-metric-for-marketplaces) The way you achieve success in a marketplace is by driving liquidity for both your supply & demand. Demand-side Liquidity = When users come to your marketplace, they can achieve their goals. Supply-side Liquidity = When supply comes to your marketplace they can achieve their goals... which are almost always to make money. If you're making a large amount of your supply-side users a full-time income, then you're helping them achieve liquidity. Now it's not so black and white and it doesn't always have to be a "full-time income." It depends what their goals are. E.g., 1) At Airbnb, renters aren't looking to quit their day jobs and become landlords full-time... they're just look to earn a substantial amount of income to offset their rent, mortgage, etc. So in this case, I would probably goal on # of renters that earn >$500 / month... and (in the first 1-5 years) try to grow this number by 10-20% MoM... and maybe by just 5% once you're in the mid-high tens of millions in yearly revenue. 2) At Kickstarter, the goal of the supply-side is to get their project successfully funded. They don't care if the project creator is "full-time"... they just want to make sure they meet their funding goal. This is why they talk about their 44% project success rate all the time - http://www.kickstarter.com/help/stats 3) At Udemy, our instructors want a substantial amount of their income to be driven from their Udemy course earnings... so we look at how many instructors are earning >$2k / month.

The best way to solve chicken and egg problems for marketplaces is to prove market need on each side independently first with a low-cost MVP-type test. Once you've proven the market on both sides with metrics it is much easier to leg in supply and demand with a strategic or enough funding to match a market on a local or niche level to ensure liquidity. For a deeper analysis, here is a post on medium that I wrote... http://bit.ly/1k2vYbY Also, feel free to schedule a call with me if you'd like to dig deeper.

You usually only recognize the commissions as revenues and use the term "Gross Merchandise Value" (GMV) to describe the size of the marketplace (value of all transactions going through the site)

Check out https://www.balancedpayments.com/ They are made for marketplaces. Airbnb CEO among others invested in them and they have some of the best pricing/payout fees. Also some good info on http://www.collaborativeconsumption.com/2013/10/08/online-marketplaces-are-hard/ One of Balanced Payments co-founders is writing this blog series on marketplaces.