What regional EU ecommerce marketplaces make sense to launch on (besides Amazon) and are there companies that assist with this.

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.

I always suggest going "uncomfortably narrow" initially so that you can really dial in the user experience and build liquidity first. Going broad will be tougher as there's too much noise to signal. Also, it's best to fake the supply side initially of you can to improve the buyers side first, then figure out supply & quality afterwards if customers are buying and you've proven out a demand strategy that will work.

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.

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.

Your support software should cater to your needs, depending on how your business operates. Fiver uses Vanilla forum and Zendesk. Thumbtack uses Zendesk. Not sure about AirBNB, their help center seems to be custom. Depending on how well funded your are, I would recommend starting with a free plan with one of the help desk SaaS products, or even using open source ticketing platform. Then, as your needs grow and you need integration with your marketplace, there's no reason you can't scale and migrate.