Why there is no online marketplace for disabiliy tools and item? Is that because the market of potential user is too small?

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.

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.

Focus on the more difficult side of the marketplace. For instance, if you think it'll be easier to get suppliers, then focus first on getting buyers - always be working on your toughest problem (aka your biggest risk). You'll find some great blogging on Marketplace and Platform topics here http://platformed.info (read the ebook too!)

Online marketplaces are typically valued by revenue, community engagement and potential. What is the company's current growth, what is the rate of growth, what is the market share potential, how it is the market, and what are the opportunities that the company affords. These all play a part in valuation. What is the reach? How many subscribers, users, etc all play a part

I'm the CTO of https://3dagogo.com a marketplace of proven to print 3D designs. We look at the two sides differently. There's not a single customer. In our case you have designers and purchasers ( sometimes the same person can be both ). Cost and methods for acquiring designers are very different than those to attract purchasers. I would clearly separate the sides and come up with separate cost structures. In my opinion when you're looking at the marketplace from the purchaser perspective, the other side's acquisition costs can be seen as fixed marketing costs.