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/