It seems that support for a marketplace could be consider a competitive advantage if it's tightly integrated into the transaction, so should this be built in house, or use a 3rd party app to do this.
What do AirBNB, Fivver, Thumbtack use?
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.
This really depends on your target audience but Zendesk allows for scalability that most startups need. You would need to gauge the level of knowledge needed for your agents - which is dependent on your startup industry.
For example, if you are a company like AirBNB, you may not need an extensive knowledgebase for your outsourced agents because the website users are not going to ask specific programming questions. However, if you are a technology company, your agents may need backend white-papers to help customers solve functionality problems themselves.
I always recommend to use a third party, and I would recommend Intercom (http://www.intercom.io/). It has instant support service to have a conversation with the users. You can always use it to create automatic messages and emails to your users, even use segmentation for those messages.
I know Airbnb uses Mixpanel, but for metric purposes, to analyze their users and their actions, but that's not exactly support: https://mixpanel.com/case-study/airbnb/
Zendesk is one of the preferred systems being used by many marketplace startups and other global brands including, say Slack. It is extensive and scalable as I have used it at advanced level.
The advantage with Zendesk is that you get an excellent support center too, for knowledgebase articles. See an example of KB that I have developed for KarmaCRM, at: http://support.karmacrm.com/hc/en-us/. See few more at: http://in23hours.com/
GrooveHQ is another good option, https://www.groovehq.com/
AirBnB are Zendesk users. They started on a free plan and kind of grew. Some of Europe's largest horizontal classifieds (double Airbnb's ticket volume), still use OTRS, an open source ticketing system.
So we work with many startups and large organizations; from my personal experience it looks like more and more startups particularly are seeking outsourced custom software solutions for their teams. Many startups are now conceived by non-technical founders who need CTOs to develop the prototype, this is where companies like Betabulls.com has been really capturing the market for custom software because of the unique problems some of these startups face - not typically off-the-shelve problems. Airbnb for example did custom web app once they pivoted but they did that themselves otherwise they would have probably hired just like many others do.
Looks like this question was 3 years old. Freshdesk has come a long way in past 3 years. I will recommend trying Freshdesk, with their new market place it opens up possibilities. If you need any help with Freshdesk, send me a note or call me.