What about them?

Arcbazar.com

It's a small US start-up organizing competitions for small-to-middle scale residential, refurbishing and interior design architecture projects. The parties involved are a crowd of international designers (not restricted to professionals), a client who initiates a competition (based on the platform's recommendations, he sets a prize amount and provides a brief complete with CAD drawings and photos), the hosting platform and a cloud of contractors eager to force their way in very early (probably with a networking subscription or fee). There is no jury, so the client picks the winners: 1st gets 60% of the prize amount, 2nd gets 30% and the 3rd gets 10%. As far as I've read the FAQ's, both registration in a competition and initiating a competition are free. The prize amount is still somehow guaranteed and the general principle is that it should be (but not compulsory) around 2% of the construction cost. So far the designers and the clients are anonymous and the briefs doesn't provide an address. Communication between the clients and the designers is exclusively through the competition platform. That probably because the 3rd party (the contractors) will have to pay for these contact details. However, not visiting the building site or the actual building in case of a refurbishment/extension is a major lack. It will deeply lower the quality of the designs and the chances for those to be implemented.
The system seems to work: for a small community library in the US, of around 1500 m2, with a prize amount of 1500 $, 84 designers from all over the world (Indonesia, Peru, Greece, Canada and so on) are registered 3 weeks before the deadline. The brief asks for common things to be submitted: plans, sections, facades, site plan and renderings, digital only. However, for a residential backyard landscape design of only 100 m2, with a prize amount of 1000$ and a brief asking for complete package suitable to send out for bids, there are only 7 registered designers, 6 weeks before the deadline.
The biggest crowd who submitted design projects was of 80 users, competing for the redevelopment of a school district in the US. Don't know the cash prize, but it's quite impressive for a start-up.
The business model (patent pending btw, which is peculiar) seems to be:
networking tax for 3rd party contractors + transaction fee (?%) for the prizes.
So far, no advertising (thumbs up!).

So, to sum up:

PROs

• Free registration for clients/designers (not yet freemium, but we'll see)

• Trustworthy (in general it seems not to be a scam, because they're providing the names and contact details for members of the team and of the board of advisors, all with an Ivy league sort of background - MIT, Harvard, etc.)
• Guaranteed prizes for the winners
• No annoying advertising / clean image of the webpage

CONs

• No real site visits are allowed. That's a major flow!
• Quite a poor overall design quality of the submissions (but the winners clearly differentiates from the others)
• No jury? No feedback at the end of the process? Just cash the prize and go? Hmm
• What about the copyrights? They become the exclusive property of the purveyor. Not so fair...
• When you ask for a complete project in the US, ready to be implemented and authorized by the local authorities, the winning designer could be a 2nd year student from Malaysia, with great ideas but with very poor actual knowledge of the construction process, details, etc
• No real exposure for the design results. Only the winning submissions are posted on Facebook.
• The cash prize is not regulated according to the complexity and difficulty of the design process. Of course, a powerful tagline of the site is 'all too often clients could not find an easy and affordable way to find competitive architectural design services', so let's offer a chance to those in need of great architecture but with no money to pay for it. But only one offending cash prize does a lot of damage to the competition platform, then to be perceived as a 99designs.com for architecture and interior design. 

So what are the reactions, apart from the on-site testimonials?

• Dwell.com  is quite harsh: “The Worst Thing To Happen To Architecture Since The Internet Started”. Probably it's all because it encourages blind design without visiting the building site. Comming from Dwell, it's BAD.
• The prestigious UK publication The Architects Journal posted a probably harsh criticism too, but unfortunately the content is for subscribers only.
• Things Magazine is skeptical, tagging Arcbazar as "reducing architectural design to a form of bartered good"

• So did Chris from IDLab , in an extensive comment on his blog, about crowdsourcing architecture. It's all about low fees and a slavery inspired business model. It's even WORST, because it will trigger the anger of the whole industry (as in the case of the above mentioned 99designs vs. the design agencies)
• Same goes with FairFees4all's comments on this blog. 

So is there anything to learn from here?
Do not offend the architecture industry by cloning the specs crowdsourcing business model. Do not be labeled as exploitation (see the comments). Or at least not if you don't get the big bucks.