Olark and other live chat systems work well when you want to provide all of your own customer support... you're basically in the middle of the transaction. This doesn't scale too well, especially when you have to query both parties to get answers and provide them. You have to become good at knowing the transaction, have good answers and information beforehand, and become good at knowing what questions to ask to reduce loops. Real time support on a marketplace this way is a challenge.

This system allows you to onboard all of the costs above onto the sellers in the marketplace.

My feeling is this would well in a marketplace where your supply side is tech-savvy. Otherwise you'll need a desktop client to keep them signed on.

Even if your sellers are tech savvy, they'll need to keep your browser window open all the time. Sure, you could build an app and push notifications, but it's hard to provide real time chat support on your mobile device. Integrating with Google Chat, Skype, etc... may be the way to go (this works well for me when using snapengage). I'd be really interesting in using this as soon as that ability is offered on the buyer-side.